Radio frequency identification (RFID) moisture tag(s) and sensors with extended sensing via capillaries

ABSTRACT

A radio frequency identification (RFID) tag includes a power harvesting circuit that operates generate power for the RFID tag from a continuous wave of a radio frequency (RF) signal. The RFID tag further includes a tuning circuit that is tuned based on a capacitance setting, where the capacitance setting is indicative of a power level of the power. The RFID tag further includes a processing module operably coupled to the tuning circuit, where the processing module operates to generate the capacitance setting to obtain a desired power level for the power.

CROSS REFERENCE TO RELATED PATENTS

The present U.S. Utility Patent Application claims priority pursuant to35 U.S.C. § 120 as a continuation of U.S. Utility application Ser. No.16/119,365, entitled “Radio Frequency Identification (RFID) MoistureTag(S) And Sensors With Extended Sensing Via Capillaries,” filed Aug.31, 2018, issuing as U.S. Pat. No. 10,734,727 on Aug. 4, 2020, which isa continuation of U.S. Utility application Ser. No. 15/443,050, entitled“Radio Frequency Identification (RFID) Moisture Tag(S) And Sensors WithExtended Sensing Via Capillaries,” filed Feb. 27, 2017, issued as U.S.Pat. No. 10,069,205 on Sep. 4, 2018, which is a continuation of U.S.Utility application Ser. No. 14/879,088, entitled “Radio FrequencyIdentification (RFID) Moisture Tag(s) And Sensors With Extended SensingVia Capillaries,” filed Oct. 8, 2015, issued as U.S. Pat. No. 9,582,981on Feb. 28, 2017, which claims priority pursuant to 35 U.S.C. § 119(e)to U.S. Provisional Application No. 62/061,257, entitled “RadioFrequency Identification (RFID) Moisture Tags and Sensors,” filed Oct.8, 2014, U.S. Provisional Application No. 62/079,369, entitled “RadioFrequency Identification (RFID) Moisture Tags and Sensors,” filed Nov.13, 2014, U.S. Provisional Application No. 62/147,890, entitled “RadioFrequency Identification (RFID) Moisture Tags and Sensors with ExtendedSensing,” filed Apr. 15, 2015, and U.S. Provisional Application No.62/195,038, entitled “RFID Moisture Tags and Sensors with ExtendedSensing via Capillaries,” filed Jul. 21, 2015.

Additionally, the above identified applications and their subject matterare expressly incorporated herein by reference in their entirety andmade part of the present U.S. Utility Patent Application for allpurposes.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates generally to sensing a detectableenvironmental condition, and, in particular, to sensing moisture usingan RFID system.

2. Description of the Related Art

In general, in an RF communication system, a single antenna structure isadapted to receive signals, the carrier frequencies (“f_(C)”) of thesesignals can vary significantly from the resonant frequency (“f_(R)”) ofthe antenna. The mismatch between f_(C) and f_(R) results in loss oftransmitted power. In some applications, this may not be of particularconcern, but, in others, such as in RF identification (“RFID”)applications, such losses are of critical concern. For example, in apassive RFID tag, a significant portion of received power is used todevelop all of the operating power required by the RFID tag's electricalcircuits. In such an application, a variable impedance circuit can beemployed to shift the f_(R) of the RFID tag's receiver so as to bettermatch the f_(C) of the transmitter of the system's RFID reader. A singledesign that is useful in all systems is precluded by the lack ofstandards as to appropriate RFID system frequencies, and, the breadth ofthe available frequency spectrum is quite broad: Low Frequency (“LF”),including 125-134.2 kHz and 140-148.f kHz; High-Frequency (“HF”) at13.56 MHz; and Ultra-High-Frequency (“UHF”) at 868-928 MHz. Compoundingthis problem is the fact that system manufacturers cannot agree on whichspecific f_(C) is the best for specific uses, and, indeed, to preventcross-talk, it is desirable to allow each system to distinguish itselffrom nearby systems by selecting different f_(C) within a defined range.

Attempts have been made to improve the ability of the RFID tag's antennato compensate for system variables, such as the materials used tomanufacture the RFID tag. However, such structural improvements, whilevaluable, do not solve the basic need for a variable impedance circuithaving a relatively broad tuning range.

Shown in FIG. 1 is an ideal variable impedance circuit 100. Circuit 100comprised of a variable inductor 102, a variable capacitor 104 and avariable resistor. When used as a tank in a resonant system, the circuit100 exhibits a quality factor (“Q”) of:

$\begin{matrix}{Q = {\frac{f_{R}}{\Delta f} = {\frac{1}{R}\sqrt{\frac{L}{C}}}}} & \lbrack 1\rbrack\end{matrix}$

where: Q=the quality factor of circuit 100;

f_(R)=the resonant frequency of circuit 100, measured in hertz;

Δf=the bandwidth of circuit 100, measured in hertz at −3 db

R=the resistance of resistor, measured in ohms;

L=the inductance of variable inductor 102, measured in henries; and

C=the capacitance of capacitor, measured in farads.

In such a system, the resonant frequency, f_(R), of circuit 100 is:

$\begin{matrix}{f_{R} = \frac{1}{2\pi\sqrt{LC}}} & \lbrack 2\rbrack\end{matrix}$

As is well known, the total impedance of circuit 100 is:

$\begin{matrix}{Z = \frac{Z_{L}Z_{C}}{Z_{L} + Z_{C}}} & \lbrack 3\rbrack\end{matrix}$

where: Z=the total impedance of circuit 100, measured in ohms;

Z_(L)=the impedance of variable inductor 102, measured in ohms; and

Z_(C)=the impedance of capacitor, measured in ohms.

As is known, the relationship between impedance, resistance andreactance is:Z=R+jX  [4]

where: Z=impedance, measured in ohms;

R=resistance, measured in ohms;

j=the imaginary unit √{square root over (−1)}; and

X=reactance, measured in ohms.

In general, it is sufficient to consider just the magnitude of theimpedance:|Z|=√{square root over (R ² +X ²)}  [5]

For a purely inductive or capacitive element, the magnitude of theimpedance simplifies to just the respective reactance's. Thus, forvariable inductor 102, the reactance can be expressed as:X _(L)=2πfL  [6]

Similarly, for capacitor, the reactance can be expressed as:

$\begin{matrix}{X_{C} = \frac{1}{2\pi\;{fC}}} & \lbrack 7\rbrack\end{matrix}$

Because the reactance of variable inductor 102 is in phase while thereactance of capacitor is in quadrature, the reactance of variableinductor 102 is positive while the reactance of capacitor is negative.Accordingly, a desired total impedance can be maintained if a change ininductive reactance is offset by an appropriate change in capacitivereactance.

Within known limits, changes can be made in the relative values ofvariable inductor 102, capacitor, and resistor to adjust the resonantfrequency, f_(R), of circuit 100 to better match the carrier frequency,f_(C), of a received signal, while, at the same, maximizing Q.

In many applications, such as RFID tags, it may be economicallydesirable to substitute for variable inductor 102 a fixed inductor 202,as in the variable tank circuit 200 shown in FIG. 2. In general, inorder to maximize Q in circuit 200.

The amplitude modulated (“AM”) signal broadcast by the reader in an RFIDsystem will be electromagnetically coupled to a conventional antenna,and a portion of the current induced in a tank circuit is extracted by aregulator to provide operating power for all other circuits. Oncesufficient stable power is available, the regulator will produce, e.g.,a power-on-reset signal to initiate system operation.

Tags based on conventional chips can be detuned by a variety of externalfactors, most commonly by proximity to liquids or metals. Such factorscan change the impedance characteristics of a tag's antenna. When thetag chip has a fixed impedance, a mismatch between the chip and theantenna results, reducing the tag's performance.

BRIEF SUMMARY OF THE INVENTION

Embodiments of the present disclosure are directed to systems andmethods that are further described in the following description andclaims. Advantages and features of embodiments of the present disclosuremay become apparent from the description, accompanying the drawings andclaims.

Accordingly, the above problems and difficulties are obviated byembodiments of the present disclosure which provide an RF-basedenvironmental moisture sensing system comprising one or more specialantenna arrangements, and an RF transceiver.

The passive radio frequency identification (RFID) moisture sensorincludes one or more antenna structures having a tail. The tail isoperable to transport a disturbance such as, but not limited to fluid ormoisture from a monitored location wherein the antenna has an impedanceand varies with proximity to the disturbance. An integrated circuitcouples to the antenna structure. This IC includes a power harvestingmodule operable to energize the integrated circuit, animpedance-matching engine coupled to the antenna, a memory module, and awireless communication module. The impedance-matching engine may vary areactive component to reduce a mismatch between the antenna impedanceand the IC and produce an impedance value (sensor code) representativeof the reactive component impedance. The memory module stores theimpedance value (sensor code) until the wireless communication modulecommunicates with an RFID reader and sends the impedance value/sensorcode to the RFID reader. The RFID reader may then determine anenvironmental condition such as the presence of moisture or fluids atthe tail of the RFID sensor and/or the magnitude (e.g. amount) of theenvironmental condition (e.g. moisture) and/or the change in suchmagnitude. This sensor may deploy several antenna and/or tails sensitiveto unique disturbances. These tails may be used to monitor differentlocations as well as different types of fluids. In one particularembodiment, the disturbance is a fluid or moisture within the gutter ofa vehicle body.

In another embodiment, the antenna arrangement includes an antennacoupled to a tail, the combination of the antenna and tail having animpedance. Further, the RF transceiver includes a number of tankcircuit(s) operatively coupled to the antenna and having a selectivelyvariable impedance. A tuning circuit is adapted to dynamically vary theimpedance of the tank circuit, and to develop a first quantized valuerepresentative of the impedance of the tank circuit, wherein the firstquantized value is a function of the modified antenna impedance.

Further embodiments provide a method for operating the first embodimentcomprising the steps of first exposing the antenna/tail to a selectedenvironmental condition such as but not limited to moisture or wetness.Next, the impedance of the tank circuit is dynamically varied tosubstantially match the modified antenna impedance. Processing the valueof the dynamically varied antenna/tail impedance is then used to sensethe environmental condition.

Another embodiment of the present disclosure provides an environmentalsensing method for use in an RF system comprising the steps of:calibrating an RF sensor by developing a first calibration valueindicative of an absence of a detectable quantity of a substance (or aknown quantity or environmental parameter) and a second calibrationvalue indicative of a presence of the detectable quantity of thesubstance (or a known quantity or environmental parameter); installingthe sensor in a structure; exposing the structure to the substance;interrogating the sensor to retrieve a sensed value; and detecting thepresence of the substance in the structure as a function of the sensedvalue relative to the first and second calibration values.

Yet another embodiment comprises multiple sensing engines that arelocated within a single integrated circuit (IC) or die that functions asa passive RFID tag. A generic sensing interface on the passive RFID tagprovides additional flexibility and expanded general sensorapplications. The present disclosure encompasses the ability for thepassive RFID tag to (or based on the data supplied by the RFID tag) tomake decisions based on multiple sensory inputs.

In yet another embodiment, the passive RFID tag/sensor includes one ormore inductive loops, wherein the inductive loop(s) have a uniqueimpedance; the unique impedance may be permanently altered in responseto an environmental parameter proximate to the inductive loop(s). Thequantized values generated in response to such an impedance change areused to indicate the occurrence of a physical event and/or the magnitudeof such an occurrence. Such events include but are not limited totemperature changes, impacts, physical damage, exposure to moisture,humidity, or contaminates.

These embodiments and additional embodiments are described in moredetails in the detailed description.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Embodiments of the present disclosure may be more fully understood by adescription of certain preferred embodiments in conjunction with theattached drawings in which:

FIG. 1 is an ideal variable impedance circuit;

FIG. 2 is a second variable impedance circuit;

FIG. 3 illustrates in block schematic form, an embodiment of aself-tuning engine;

FIG. 4 illustrates in flow diagram form the sequencing of operations inthe self-tuning engine shown in FIG. 3;

FIG. 5 illustrates in block schematic form, another embodiment of aself-tuning engine;

FIG. 6 illustrates, in block diagram form, an RF receiver circuit havinga field strength detector constructed in accordance with an embodimentof the present disclosure;

FIG. 7 illustrates, in block diagram form, a field strength detectorcircuit constructed in accordance with an embodiment of the presentdisclosure;

FIG. 8 illustrates, in block schematic form, a more detailed embodimentof the field strength detector circuit shown in FIG. 7;

FIG. 9 illustrates, in flow diagram form, the sequencing of operationsin the field strength detector circuit shown in FIG. 5;

FIG. 10 illustrates, in graph form, the response of the field strengthdetector circuit shown in FIG. 6 to various conditions;

FIG. 11 illustrates, in block schematic form, an RF receiver circuitconstructed in accordance with another embodiment of the presentdisclosure;

FIG. 12 illustrates, in block schematic form, an alternativerepresentation of the impedance represented by the antenna and the tankcircuit of the exemplary RFID receiver circuit;

FIG. 13 illustrates, in block schematic form, an alternative exemplaryembodiment of the field strength detector circuit shown in FIG. 8;

FIG. 14 illustrates, in block schematic form, an alternative exemplaryembodiment of the field strength detector circuit shown in FIG. 8

FIG. 15 illustrates, in block schematic form, an exemplary RFIDsub-system containing tag and reader;

FIG. 16 illustrates, in flow diagram form, the sequencing of theoperations in developing a reference table associating tank tuningparameters with system frequency;

FIG. 17, comprising FIGS. 17A and 17B, illustrates an RF systemconstructed in accordance with one embodiment of the present disclosureto sense environmental conditions in a selected region surrounding thesystem;

FIG. 18 illustrates, in perspective, exploded view, one possibleconfiguration of an antenna and tail arrangement adapted for use in thesystem of FIG. 17;

FIG. 19 illustrates, in flow diagram form, the sequencing of theoperations in detecting the presence of a contaminant using, e.g., theantenna of in the system shown in FIG. 18;

FIG. 20 is a block diagram of a RFID wireless solution provided byembodiments of the present disclosure;

FIG. 21 is a block diagram of one arrangement of smart sensors and adata processing unit in accordance with embodiments of the presentdisclosure;

FIG. 22 provides an illustration of an antenna arrangement in accordancewith embodiments of the present disclosure;

FIG. 23 is a view of an RFID moisture or humidity sensing tag inaccordance with an embodiment of the present disclosure;

FIG. 24 is a view of a folded RFID tag comprising a radiating element inaccordance with an embodiment of the present disclosure;

FIGS. 25A-C are block diagrams of arrangements of self-tuning engines tosupport the reporting of several stimuli with multiple passive RFIDsensors using antenna impedance sensing mechanisms in accordance withembodiments of the present disclosure;

FIG. 26 is a block diagram of a self-tuning engine in accordance withembodiments of the present disclosure;

FIG. 27 illustrates an embodiment of the self-tuning engine provided byembodiments of the present disclosure along with the varactors that aredriven by the self-tuning circuitry (also referred to as MMS engine inthis disclosure);

FIG. 28 provides a flow chart of one embodiment of the presentdisclosure; and

FIGS. 29A and 29B illustrate one passive RFID sensor in accordance withan embodiment of the present disclosure;

FIGS. 30-38 are graphs of several measurements taken using two differentsamples (sample 1 and sample 2) while on a human wrist and on a 2.5 inchdiameter water bottle in different configurations in accordance withembodiments of the present disclosure;

FIG. 39 is a graph of the WristTag capacitance variation vs. air gap inaccordance with embodiments of the present disclosure;

FIGS. 40, 41 and 42 show plots of the internal currents in the antennawith 10 mm gap, 5 mm gap and no gap between antenna and wrist inaccordance with embodiments of the present disclosure;

FIG. 43 is a diagram of an embodiment of a diaper tag/sensor inaccordance with an embodiment of the present disclosure;

FIG. 44 is a plot of average sensitivity as water is added for variousfrequency ranges in accordance with embodiments of the presentdisclosure;

FIG. 45 is a plot of average sensor code as water is added for variousfrequency ranges in accordance with embodiments of the presentdisclosure;

FIG. 46 is a plot of showing the code shift magnitude from a drycondition vs. amount of water added for the various frequency ranges inaccordance with embodiments of the present disclosure;

FIGS. 47, 48 and 49 show results of behavior across the 902-928 MHzfrequency range and exhibit very large code shifts making the additionof water very detectable by RFID tag/sensor in accordance withembodiments of the present disclosure;

FIGS. 50, 51 and 52 show results where the RFID tag was mounted with thecenter 120 mm from the edge of the Super-Absorbent Polymer (SAP) fillerwhich is where the water is added to the diaper in accordance withembodiments of the present disclosure;

FIG. 53 is a plot of the read range for the hand-held reader inaccordance with embodiments of the present disclosure;

FIG. 54 is a graph showing Antenna behavior at air/water interface (tophalf labeled 1 is air bottom half labeled 2 is water) in accordance withembodiments of the present disclosure;

FIGS. 55A and 55B are a set of plots showing sensor code spread vs. timefor various frequency ranges and various wetness level (30 ml, 60 ml,120 ml and 240 ml shown top left, top right, bottom left and bottomright, respectively) in accordance with embodiments of the presentdisclosure;

FIG. 56 is a set of plots showing similar to experiments with a 6 mmfoam spacer added between the diaper and the jug in accordance withembodiments of the present disclosure;

FIG. 57 is another embodiment of the RFID tag/sensor of the presentdisclosure;

FIG. 58 is a set of plots showing average tag sensitivity across theNorth American band in accordance with embodiments of the presentdisclosure;

FIG. 59 is a schematic of yet another embodiment a moisture tag antennadesign in accordance with embodiments of the present disclosure;

FIG. 60 is a table comparing an embodiment a moisture tag antenna designwith a prior art design in accordance with embodiments of the presentdisclosure;

FIG. 61 is a schematic of yet another embodiment a moisture tag antennadesign in accordance with embodiments of the present disclosure;

FIGS. 62, 63, 64 and 65 are schematics of yet other embodiments of amoisture tag antenna design where these antenna can be used with, forexample, the IC of FIG. 20 in accordance with embodiments of the presentdisclosure;

FIGS. 66, 67 and 68 show the current distribution for FIGS. 64, 63 and65, respectively;

FIG. 69 shows a Moisture Tag which is a conventional compact dipole withadded interdigitated capacitor in accordance with embodiments of thepresent disclosure;

FIGS. 70 and 71 are diagrams of yet another embodiment of the wickingtails in accordance with embodiments of the present disclosure;

FIG. 72 is a diagram used to describe further experimentation on wickingmaterials within a car trunk (gutter of a vehicle body) involving anincline in accordance with embodiments of the present disclosure.

In the drawings, similar elements will be similarly numbered wheneverpossible. However, this practice is simply for convenience of referenceand to avoid unnecessary proliferation of numbers, and is not intendedto imply or suggest that the present disclosure requires identity ineither function or structure in the several embodiments.

DETAILED DESCRIPTION OF THE INVENTION

Preferred embodiments of the present invention are illustrated in theFIGs., like numerals being used to refer to like and corresponding partsof the various drawings.

Throughout this description, the terms assert and negate may be usedwhen referring to the rendering of a signal, signal flag, status bit, orsimilar apparatus into its logically true or logically false state,respectively, and the term toggle to indicate the logical inversion of asignal from one logical state to the other. Alternatively, reference maybe made to the mutually exclusive Boolean states as logic_0 and logic_1.Of course, as is well known, consistent system operation can be obtainedby reversing the logic sense of all such signals, such that signalsdescribed herein as logically true become logically false and viceversa. Furthermore, it is of no relevance in such systems which specificvoltage levels are selected to represent each of the logic states.

Embodiments of the present disclosure provide various passive radiofrequency identification (RFID) sensors. These passive RFID moisturesensors include an antenna coupled to a tail, a processing module, and awireless communication module. The antenna and coupled tail have animpedance that may vary with an environment in which the antenna/tail isplaced. The processing module couples to the antenna and has one or moreself-tuning module(s) that may vary a reactive component impedancecoupled to the antenna in order to change a system impedance. The systemimpedance including both the antenna impedance, tail impedance and thereactive component impedance. The self-tuning module(s) then produces animpedance value representative of the reactive component impedance. Amemory module may store the impedance value which may then later iscommunicated to an RFID reader via the wireless communication module.The RFID reader then exchanges the impedance value representative of thereactive components of impedance with the RFID reader such that the RFIDreader or another external processing unit may process the impedancevalue in order to determine environmental conditions at the antenna.These environmental conditions may include but are not limited totemperature, humidity, wetness, or proximity of the RFID reader to thepassive RFID sensor.

In this disclosure, there will be references to several commonly usedmaterial types that RFID tags are affixed to for the purposes ofdetection and sensing. Some of the acronyms used are:

-   -   a. PTFE: Polytetrafluoroethylene (e.g. Teflon by DuPont)    -   b. PMMA: Poly(methyl methacrylate) (lightweight or        shatter-resistant alternative to glass)    -   c. PET: Polyethylene terephthalate (a thermoplastic polymer        resin of the polyester family and is used in bottles; synthetic        fibers; beverage, food and other liquid containers;        thermoforming applications; and engineering resins often in        combination with glass fiber, e.g. Dacron, Terylene Lavsan).

In this disclosure, the terms sensitivity or tag sensitivity, is used.Tag sensitivity is specified as the minimum power arriving at the tagnecessary to communicate with the tag. The setup to measure thesensitivity consists of a reader, antenna, and the tag under test. Themeasurement proceeds by starting with a high power output from thereader and communicating with the tag. The power is then reduced untilthe tag stops communicating, so the minimum reader power to communicateis then known. This can generally be done with a binary search to speedthings up. The process is repeated for each RFID channel.

The minimum reader power to communicate is not the tag sensitivity. Ifthe reader's antenna is placed farther away, then more reader power isneeded. Or, if a higher gain antenna is substituted, then lower readerpower is needed. To get the tag sensitivity, the minimum reader power isadjusted to find the power arriving at the tag, and this power is thetag sensitivity:tag sensitivity=minimum reader power+cable losses+antenna gain+spacelosswhere space loss takes into account the fact that the power densitydecreases as 1/r². This is a relatively simple spreadsheet calculationthat is well known in the art, and tools that support tag sensitivitymeasurements just have boxes where you fill in the cables losses,antenna gain, and distance from the antenna to the tag. The softwarethen does the binary search for the minimum reader power, and it knowsthe frequency it is using, then it makes the needed adjustments andplots tag sensitivity vs. frequency.

FIG. 3 illustrates in block schematic form, an embodiment of aself-tuning engine. In FIG. 3 the amplitude modulated (“AM”) signalbroadcast by the reader in an RFID system will be magnetically coupledto a conventional coil antenna comprising inductor 304′, and a portionof the induced current is extracted via nodes 308 and 310 by a regulator316 to produce operating power for all other circuits. Once sufficientstable power is available, regulator 316 will produce a PowerOK signalto initiate system operation (see, 402 and 404 in FIG. 4). If desired, avariable resistor (not shown) can be provided in parallel with inductor304′, generally between nodes 308 and 310, and regulator 316 can beconstructed so as to automatically vary this resistance to control thegain of the tank circuit 302′.

In response to the PowerOK signal, a timer 322 will periodicallygenerate a timing pulse t (see, generally, 406, 408, 410, and 412 inFIG. 4). Preferably, the frequency of t pulses is a selectedsub-multiple of the received signal, and the duty cycle is on the orderof fifty percent (50%). However, as will be explained below, other dutycycles may be appropriate depending on the specific circuit elementsselected to implement my invention.

In response to the PowerOK signal, a reference voltage generator 332will continuously produce a reference voltage signal V_(Ref)proportional to the voltage induced by the received signal between nodes308 and 310. In response to the assertion of each t pulse, adifferentiator 334 will save the then-current value of the V_(Ref)signal (see, 414 in FIG. 4). Thereafter, differentiator 334 willcontinuously determine the polarity of the change of the previouslysaved value with respect to the then-current value of the V_(Ref) signal(see, 416 in FIG. 4). If the polarity is negative, indicating that thecurrent V_(Ref) signal is lower than the previously-saved V_(Ref)signal, differentiator 334 will assert a change direction signal;otherwise, differentiator 334 will negate the change direction signal(see, 418 in FIG. 4).

In response to each negation of t, a direction selector 342 will togglebetween an up state and a down state if and only if differentiator 334is then asserting the change direction signal; otherwise, selector 342continues to maintain its current state (see, 420 in FIG. 4).

In response to the PowerOK signal, a ramp generator 346 will reset to apredetermined initial value (see, 404 in FIG. 4). Thereafter, inresponse to each assertion of t, generator 346 will selectively changethe value of capacitor 306, thereby changing the resonant frequencyf_(R) of circuit 302′ (see, 422 in FIG. 4). Preferably, the initialvalue for generator 346 is selected such that the initial resonantfrequency f_(R) of circuit 302′ will approximate the anticipated carrierfrequency f_(C) of the received signal, thereby assuring convergencewith a minimal number of re-tuning cycles. Although the initial valuecan be established using any of several known non-volatile techniques,including hard wiring or any of a variety of read-only-memory (ROM)structures, re-writable mechanisms, such as a flash or otherelectrically-programmable ROM structure are preferable. Using thelatter, it would be a simple matter to construct regulator 316 so as toprovide a PowerLoss signal when the level of available power drops to apredetermined minimum, and then, in response to the PowerLoss signal, tocopy the current value in generator 346 into the memory. Upon nextreceiving the PowerOK signal, the generator 346 will resume operation atthe stored value, potentially reducing convergence time.

After each change in the resonant frequency f_(R) of circuit 302′,circuit 312 again determines the polarity of change of V_(Ref). If thepolarity is found to be positive, the resonant frequency f_(R) isconverging toward the carrier frequency f_(C), so the direction ofchange is correct. However, if the polarity is found to be negative, theresonant frequency f_(R) is diverging from the carrier frequency, andthe direction of change must be reversed. During operation, circuit 312will selectively vary the value of capacitor 306 so that the resonantfrequency f_(R) of tank circuit 302′ converges toward the carrierfrequency f_(C) of the received signal. Thus, if the polarity is foundto be positive, circuit 312 will continue to vary the value of capacitor306 in the currently-selected direction, say, for example, “up”; but, ifthe polarity is found to be negative, circuit 312 will switch thedirection in which the value of capacitor 306 is varied, i.e., from “up”to “down”, and begin varying the value of capacitor 6 in thenewly-selected direction, now “down”. In this manner, circuit 312 isable to converge the resonant frequency f_(R) toward the carrierfrequency f_(C) regardless of whether or not the resonant frequency isinitially higher or lower than the carrier frequency.

In the current embodiment, it is irrelevant which direction is initiallyselected by selector 342, as circuit 312 will quickly detect divergenceand reverse the state of selector 342. However, if desired, apredetermined initial direction can be selected during initializationusing conventional means.

It is to be expected that, as difference between the resonant frequencyf_(R) of tank circuit 302′ and the carrier frequency f_(C) of thereceived signal becomes relatively small, the ability of differentiator334 to detect polarity changes will be significantly diminished. At suchtime, circuit 312 will tend to seek, i.e., changing tuning direction oneach t. Additional circuitry could be easily added to detect thiscondition and to, for example, significantly decrease the operatingfrequency of timer 322 or, if desired, cease operation.

Another embodiment of a self-tuned engine that is digitally based isshown in FIG. 5. Thus, for example, in the digital circuit 500 shown inFIG. 5, timer 322 could comprise a clock 502 and an up/down-counter 504adapted to continuously negate the t signal while down-counting topredetermined minimum value and then to continuously assert the t signalwhile up-counting to a predetermined maximum value, the counter 504automatically reversing count direction upon reaching the predeterminedminimum/maximum values. V_(Ref) generator 332 could be implemented usinga full-wave rectifier 506 and a low-pass filter 508, whiledifferentiator 334 could comprise a comparator 510 with its positiveinput adapted to receive the current value of V_(Ref) and its negativeinput adapted to receive the previous value of V_(Ref) captured andsaved by a sample-and-hold 512. Finally, selector 342 can be a simpletoggle latch 514, while generator 346 could be an n-bit, bidirectionaledge-triggered shift register 516. In response to the assertion of thePowerOK signal, shift register 516 will preferably initialize thehigh-order half of the n-bits to logic_0, and the low-order half tologic_1; in response to the leading-edge of the t signal (i.e., uponeach assertion of t), shift register 516 will shift either left orright, depending on the state of toggle latch 514. Thus, to increasefrequency, register 516 would perform a right-shift with a left fill oflogic_0; whereas to decrease frequency, register 516 would perform aleft-shift with a right-fill of logic_1.

When comparator 510 negates the change direction signal, the resonantfrequency of circuit 302″ is converging on the carrier frequency of thereceived signal; whereas, when comparator 510 asserts the changedirection signal, the resonant frequency of circuit 302″ is divergingfrom the carrier frequency of the received signal. Thus, for example, ifthe old value held in sample-and-hold 512 is less than the new valueprovided by the filter 508, comparator 510 will negate the changedirection signal, indicating that register 516 is shifting in thecorrect direction to achieve convergence; under this condition, toggle514 will not toggle. On the other hand, if the old value held insample-and-hold 512 is greater than the new value provided by the filter508, comparator 510 will assert the change direction signal, indicatingthat register 516 is not shifting in the correct direction to achieveconvergence; under this condition, toggle 514 will toggle.

In the embodiment shown in FIG. 5, it is preferable but not necessary toselect the minimum anticipated settling time of the sample-and-hold 512as the minimum duration of the negated portion of each t pulse. For theperiod of t, it is preferable but not necessary to select the minimumanticipated settling time of the tank circuit 302′ to each variation intank capacitance. In such an arrangement, the negated portion of each tpulse will be relatively small with respect to the asserted portion. Ingeneral, this arrangement should enable circuit 500 to “re-tune” thetank circuit 302′ as quickly as the various circuit components are ableto detect, and then respond to, the resulting changes in V_(Ref).

Conventional dipole design for RFID tags use a small inductive loop totune out the input capacitance of the RFID IC. By altering a conductivematerial near this inductive tuning loop, the inductance depends on theproperties of the conductive material. The self-tuning engine detectsthe change in inductance and adjusts its input capacitance to maintainpeak power to the die. The change in capacitance can be read from thedie as a sensor code using the standard EPC read command. The sensorcode reflects the change in the conductive material proximate to theinductive loop.

In general, prior disclosures have focused primarily on quantizing thevoltage developed by the tank circuit as the primary means of matchingthe f_(R) of the tank circuit to the transmission frequency, f_(C), ofthe received signal. However, this voltage quantization is, at best,indirectly related to received signal field strength. Other effectiveand efficient methods may quantize the received field strength as afunction of induced current. In particular, a method and apparatusadapted to develop this field quantization in a form and manner that issuitable for selectively varying the input impedance of the receivercircuit to maximize received power, especially during normal systemoperation. Additionally, in light of the power sensitive nature of RFIDsystems, disclosed methods and apparatus of the present disclosure varythe input impedance with a minimum power loss.

While prior disclosures use methods to sense environmental changes towhich the RFID tag is exposed, the present disclosure, illustratedthrough various embodiments, provides for new sensing methods adapted tooperate in a variety of environments that are more efficient, compact,adaptable and self-tuning

Shown in FIG. 6 is an RF receiver circuit 600 suitable for use in anRFID application. Note that the Tank 604, Tuner 606 and Regulator 608can be embodied by the structures shown in FIG. 3 or FIG. 5. An RFsignal electromagnetically coupled to an antenna 602 is received via atank circuit 604, the response frequency, f_(R), of which is dynamicallyvaried by a tuner 606 to better match the transmission frequency, f_(C),of the received RF signal, thus obtaining a maximum power transfer. Inparticular, the RMS voltage induced across the tank circuit 604 by thereceived RF signal is quantized by tuner 606 and the developedquantization employed to control the impedance of the tank circuit 604as explained above. Also, the unregulated, AC current induced in thetank circuit by the received RF signal is conditioned by a regulator 608to provide regulated DC operating power to the receiver circuit 600.This allows the tank circuit 604 to function as a power harvestingcircuit wherein the power may be stored in a capacitor, charge pump orother like circuit. In accordance with our present disclosure, we nowprovide a field strength detector 610, also known as a power detector,adapted to develop a field-strength value as a function of the fieldstrength of the received RF signal. As indicated in FIG. 6, fieldstrength detector 610 is adapted to cooperate with the regulator 608 inthe development of the field-strength value. Field strength detector 610can be adapted to cooperate with the tuner 606 in controlling theoperating characteristics of the tank circuit 604.

In general, in an RF communication system, an antenna structure is usedto receive signals, the carrier frequencies (“f_(C)”) of which may varysignificantly from the natural resonant frequency (“f_(R)”) of theantenna. It is well known that mismatch between f_(C) and f_(R) resultsin loss of transmitted power. In some applications, this may not be ofparticular concern, but, in others, such as in RF identification(“RFID”) applications, such losses are of critical concern. For example,in a passive RFID tag, a significant portion of received power is usedto develop all of the operating power required by the RFID tag'selectrical circuits. In such an application, a variable impedancecircuit may be employed to shift the f_(R) of the RFID tag's receiver soas to better match the f_(C) of the transmitter of the system's RFIDreader.

In accordance with one embodiment of the present disclosure, theamplitude modulated (“AM”) signal broadcast by the reader in an RFIDsystem (or other CW source) are magnetically coupled to a conventionalcoil antenna comprising inductor, and a portion of the induced currentis extracted via nodes by a regulator to produce operating power for allother circuits. Such a regulator may include a charge pump.

Shown by way of example in FIG. 7 is one possible embodiment of a fieldstrength or power detector 700 (field strength detector 610 of FIG. 6)that is integrated into the sensor. This embodiment employs a shunt-typeregulator 702 so that, during normal operation, the shunted ‘excess’current can be used as a reference against which we develop thefield-strength value. In this regard, reference module 704 produces ashunt current reference value proportional to the shunted current, andthen develops a mirrored current reference value as a function of boththe shunted current and a field strength reference current provided by adigitally-controlled current source 706. Preferably, once the tuner 606has completed its initial operating sequence, whereby the f_(R) of thetank circuit 604 has been substantially matched to the f_(C) of thereceived signal, a digital control 708 initiates operation of thecurrent source 706 at a predetermined, digitally-established minimumfield strength reference current. After a predetermined period of time,control 708 captures the mirrored current reference value provided bythe current reference module 704, compares the captured signal against apredetermined threshold value, and, if the comparison indicates that thefield strength reference current is insufficient, changes, in accordancewith a predetermined sequence of digital-controlled increments, thefield strength reference current; upon the comparison indicating thatthe field strength reference current is sufficient, control 708 will, atleast temporarily, cease operation.

In accordance with embodiments of the present disclosure, the digitalfield-strength value developed by control 708 to control the fieldstrength current source 706 is a function of the current induced in thetank circuit 604 by the received RF signal. Once developed, this digitalfield-strength value can be employed in various ways. For example, itcan be selectively transmitted by the RFID device (using conventionalmeans) back to the reader (not shown) for reference purposes. Such atransaction can be either on-demand or periodic depending on systemrequirements. One embodiment distributes a plurality of RFID tagdevices, perhaps randomly, throughout a restricted, 3-dimensional space,e.g., a loaded pallet. The reader is programmed to query, at an initialfield strength, all tags “in bulk” and to command all tags that havedeveloped a field-strength value greater than a respectivefield-strength value to remain ‘silent’. By performing a sequence ofsuch operations, each at an increasing field strength, the reader will,ultimately, be able to isolate and distinguish those tags most deeplyembedded within the space; once these ‘core’ tags have been read, areverse sequence can be performed to isolate and distinguish all tagswithin respective, concentric ‘shells’ comprising the space of interest.Although, in all likelihood, these shells will not be regular in eithershape or relative volume, the analogy should still be applicable.

FIG. 8 illustrates one embodiment of a field strength detector 800. Ingeneral, shunt circuit 802 develops a substantially constant operatingvoltage level across supply node 804 and ground node 806. Shuntregulators of this type are well known in the art, and typically usezener diodes, avalanche breakdown diodes, diode-connected MOS devices,and the like.

As can be seen, current reference 704 of FIG. 7 may be implemented inthe form of a current mirror circuit 808, connected in series with shuntcircuit 802 between nodes 804 and 806. As is typical, current mirrorcircuit 808 comprises a diode-connected reference transistor 810 and amirror transistor 812. If desired, a more sophisticated circuit such asa Widlar current source may be used rather than this basictwo-transistor configuration. For convenience of reference, the currentshunted by shunt circuit 802 via reference transistor 810 is designatedas i_(R); similarly, the current flowing through mirror transistor 812is designated as i_(R)/N, wherein, as is known, N is the ratio of thewidths of reference transistor 810 and mirror transistor 812.

Here, the field strength current source 816 is implemented as a set of nindividual current sources, each connected in parallel between thesupply node 804 and the mirror transistor 812. In general, fieldstrength current source 816 is adapted to source current at a levelcorresponding to an n-bit digital control value developed by a counter818. In the illustrated embodiment, wherein n=5, field strength currentsource 816 is potentially capable of sourcing thirty-two distinctreference current levels. We propose that the initial, minimum referencecurrent level be selected so as to be less than the current carryingcapacity of the mirror transistor 812 when the shunt circuit 802 firstbegins to shunt excess induced current through reference transistor 812;that the maximum reference current level be selected so as to be greaterthan the current carrying capacity of the mirror transistor 812 when theshunt circuit 802 is shunting a maximum anticipated amount of excessinduced current; and that the intermediate reference current levels bedistributed relatively evenly between the minimum and maximum levels. Ofcourse, alternate schemes may be practicable, and, perhaps, desirabledepending on system requirements.

Within control 818, a conventional analog-to-digital converter (“ADC”)820, having its input connected to a sensing node 814, provides adigital output indicative of the field strength reference voltage,v_(R), developed on sensing node 814. In one embodiment, ADC 820 maycomprise a comparator circuit adapted to switch from a logic_0 state toa logic_1 when sufficient current is sourced by field strength currentsource 816 to raise the voltage on sensing node 814 above apredetermined reference voltage threshold, v_(th). Alternatively, ADC820 may be implemented as a multi-bit ADC capable of providing higherprecision regarding the specific voltage developed on sensing node 814,depending on the requirements of the system. Sufficient current may becharacterized as that current sourced by the field strength currentsource 816 or sunk by mirror transistor 812 such that the voltage onsensing node 814 is altered substantially above or below a predeterminedreference voltage threshold, v_(th). In the exemplary case of a simpleCMOS inverter, v_(th) is, in its simplest form, one-half of the supplyvoltage (VDD/2). Those skilled in the art will appreciate that v_(th)may by appropriately modified by altering the widths and lengths of thedevices of which the inverter is comprised. In the exemplary case amulti-bit ADC, v_(th) may be established by design depending on thesystem requirements and furthermore, may be programmable by the system.

In the illustrated embodiment, a latch 822 captures the output state ofADC 820 in response to control signals provided by a clock/controlcircuit 824. If the captured state is logic_0, the clock/control circuit824 will change counter 818 to change the reference current beingsourced by field strength current source 816; otherwise clock/controlcircuit 824 will, at least temporarily, cease operation. However,notwithstanding, the digital field-strength value developed by counter818 is available for any appropriate use, as discussed above.

The present disclosure also provides a method and apparatus for aself-tuning engine with, optionally, the ability to detect RF fieldstrength for use generally in RFID tags and sensors. A field strengthreference generator develops a field strength reference current as afunction of a field strength of a received RF signal; and a fieldstrength quantizer develops a digital field strength value indicative ofthe field strength reference current. In one embodiment, detected fieldstrength is used to dynamically vary the impedance of a tank circuit viaan optimization loop that includes a search process whereby, over time,induced current is maximized. A similar process, as explained above isused for the self-tuning engine. Incorporating dithering into theprocess will be further discussed with reference to FIGS. 26 and31A-31D.

By way of example, FIG. 9 illustrates one possible general operationalflow of a field strength detector in accordance with embodiments of thepresent disclosure. Upon activation, counter 818 is set to its initialdigital field-strength value (step 902), thereby enabling field strengthcurrent source 816 to initiate reference current sourcing at theselected level. After an appropriate settling time, the field strengthreference voltage, v_(R), developed on sensing node 814 and digitized byADC 820 is captured in latch 822 (step 904). If the captured fieldstrength reference voltage, v_(R), is less than (or equal to) thepredetermined reference threshold voltage, v_(th), clock/control 824will change counter 818 (step 906). This process will repeat, changingthe reference current sourced by field strength current source 816 untilthe captured field strength reference voltage, v_(R), is greater thanthe predetermined reference threshold voltage, v_(th), (at step 908), atwhich time the process will stop (step 910). As illustrated, this sweepprocess can be selectively reactivated as required, beginning each timeat either the initial field-strength value or some other selected valuewithin the possible range of values as desired.

The graph provided in FIG. 10 depicts several plots of the voltagedeveloped on sensing node 814 as the field strength detector circuit 700sweeps the value of counter 818 according to the flow illustrated inFIG. 9. As an example, note that the curve labeled “A” in FIG. 10 beginsat a logic_0 value when the value of counter 818 is at a minimum valuesuch as “1” as an exemplary value. Subsequent loops though the sweeploop gradually increase the field strength reference voltage on sensingnode 814 until counter 818 reaches a value of “4” as an example. At thispoint, the “A” plot in FIG. 10 switches from a logic_0 value to alogic_1 value, indicating that the field strength reference voltage,v_(R), on sensing node 814 has exceeded the predetermined referencethreshold voltage, v_(th). Other curves labeled “B” through “D” depictincremental increases of reference currents, i_(R), flowing throughreference device, resulting in correspondingly higher mirrored currentsflowing through the mirror device. This incrementally higher mirrorcurrent requires field strength current source to source a highercurrent level which in turn corresponds to higher values in counter 818.Thus, it is clear that embodiments of the present disclosure are adaptedto effectively and efficiently develop a digital representation of thecurrent flowing through sensing node 814 that is suitable for anyappropriate use.

One such use, as discussed earlier, of field strength detector 610 is tocooperate with tuner 606 in controlling the operating characteristics ofthe tank circuit 604. FIG. 11 illustrates one possible embodiment wherereceiver circuit 1100 uses a field strength detector 1102 speciallyadapted to share with tuner 1104 the control of the tank circuit 1106.Dynamically tuning, via tuner 1104, the tank circuit 1106 allows one todynamically shift the f_(R) of the tank circuit 1106 to better match thef_(C) of the received RF signal at antenna 1108. FIG. 11 adds amultiplexer 1110 to tuner 1104 to facilitate shared access to the tunercontrol apparatus. Shown in FIG. 4 is the operational flow of fieldstrength detector 1100 upon assuming control of tank circuit 1106.

In context of this particular use, once tuner 1104 has completed itsinitial operating sequences, and field strength detector 1100 hasperformed an initial sweep (as described above and illustrated in FIG.4) and saved in a differentiator 1112 a base-line field-strength valuedeveloped in counter 1114, clock/control 1116 commands multiplexer 1110to transfer control of the tank circuit 1106 to field strength detector1102 (all comprising step 404 in FIG. 4). Upon completing a secondcurrent sweep, differentiator 1112 will save the then-currentfield-strength value developed in the counter 1114 (step 414).Thereafter, differentiator 1112 will determine the polarity of thechange of the previously saved field-strength value with respect to thethen-current field-strength value developed in counter 1114 (step 416).If the polarity is negative (step 418), indicating that the currentfield-strength value is lower than the previously-saved field-strengthvalue, differentiator 1112 will assert a change direction signal;otherwise, differentiator 1112 will negate the change direction signal(step 420). In response, the shared components in tuner 1104 downstreamof the multiplexer 1110 will change the tuning characteristics of tankcircuit 1106 (step 422). Now, looping back (to step 414), the resultingchange of field strength, as quantized is the digital field-strengthvalue developed in counter 1114 during the next sweep (step 414), willbe detected and, if higher, will result in a further shift in the f_(R)of the tank circuit 1106 in the selected direction or, if lower, willresult in a change of direction (step 420). Accordingly, over a numberof such ‘seek’ cycles, embodiments of the present disclosure willselectively allow the receiver 1100 to maximize received field strengtheven if, as a result of unusual factors, the f_(R) of the tank circuit1106 may not be precisely matched to the f_(C) of the received RFsignal, i.e., the reactance of the antenna is closely matched with thereactance of the tank circuit, thus achieving maximum power transfer. Inan alternative embodiment, it would be unnecessary for tuner 1104 toperform an initial operating sequence. Rather, field strength detector1102 may be used exclusively to perform both the initial tuning of thereceiver circuit 1100 as well as the subsequent field strengthdetection. Note that the source impedance of antenna 1108 and loadimpedance of tank circuit 1106 may be represented alternatively inschematic form as in FIG. 12, wherein antenna 1108 is represented asequivalent source resistance R_(S) 1202 and equivalent source reactanceX_(S) 1204, and tank circuit 1106 is represented as equivalent loadresistance R_(L) 1206 and equivalent, variable load reactance X_(L)1208.

In another embodiment, embodiments of the present disclosure may beadapted to sense the environment to which a tag is exposed, as well assensing changes to that same environment. The auto-tuning capability oftuner 606 acting in conjunction with tank circuit 604 detects antennaimpedance changes (in addition to the embodiments illustrated in FIG. 3,FIG. 4, and FIG. 5). These impedance changes may be a function ofenvironmental factors such as proximity to interfering substances, e.g.,metals or liquids, as well as a function of a reader or receiver antennaorientation. Likewise, as disclosed herein, field strength (i.e.,received power) detector may be used to detect changes in received power(i.e., field strength) as a function of, for example, power emitted bythe reader, distance between tag and reader, physical characteristics ofmaterials or elements in the immediate vicinity of the RFID tag andreader, or the like. Sensing the environment or, at least, changes tothe environment is accomplished using one or both of these capabilities.

As an example, the RFID tag 1500 of FIG. 15, contains both a source tagantenna (not shown, but see, e.g., FIG. 8) and a corresponding load chiptank circuit 604 (not shown, but see, e.g., FIG. 8). Each contains bothresistive and reactive elements as discussed previously (see, e.g., FIG.12). Tag 1500 containing such a tank circuit 604 mounted on a metallicsurface will exhibit antenna impedance that is dramatically differentthan the same tag 1500 in free space or mounted on a container ofliquid. Shown in Table 1 are exemplary values for impedance variationsin both antenna source resistance 1202 as well as antenna sourcereactance 1204 as a function of frequency as well as environmentaleffects at an exemplary frequency.

TABLE 1 Antenna Impedance Variations 860 MHz 870 MHz 880 MHz 890 MHz Rs,□ Xs, □ Rs, □ Xs, □ Rs, □ Xs, □ Rs, □ Xs, □ In Air 1.3 10.7 1.4 10.9 1.511.2 1.6 11.5 On Metal 1.4 10.0 1.5 10.3 1.6 10.6 1.7 10.9 On Water 4.911.3 1.8 11.1 2.4 11.7 2.9 11.5 On Glass 1.8 11.1 2.0 11.4 2.2 11.7 2.512.0 On Acrylic 1.4 10.6 1.6 11.1 1.7 11.4 1.9 11.7 900 MHz 910 MHz 920MHz 930 MHz Rs, □ Xs, □ Rs, □ Xs, □ Rs, □ Xs, □ Rs, □ Xs, □ In Air 1.811.8 2.0 12.1 2.2 12.4 2.4 12.8 On Metal 1.9 11.2 2.1 11.6 2.3 12.0 2.612.4 On Water 2.5 12.3 3.0 12.7 5.8 14.1 9.1 13.2 On Glass 2.8 12.4 3.212.8 3.7 13.2 4.2 13.6 On Acrylic 2.0 12.1 2.3 12.4 2.5 12.8 2.8 13.2

The tuner circuit 606 of embodiments of the present disclosureautomatically adjusts the load impendence by adjusting load reactance1208 to match source antenna impedance represented by source resistance1202 and source reactance 1204. As previously disclosed, matching of thechip load impedance and antenna source impedance can be performedautomatically in order to achieve maximum power transfer between theantenna and the chip. A digital shift register 1502 allows selectivelychanging the value of the load reactive component 1208 (see, e.g., FIG.12), in the present case a variable capacitor, until power transfer ismaximized. This digital value of the matched impendence may be usedeither internally by the RFID tag 1500, or read and used by the reader1504, to discern relative environmental information to which the RFIDtag 1500 is exposed. For example, tag 1500 may contain a calibratedlook-up-table within the clock/control circuit 824 which may be accessedto determine the relevant environmental information. Likewise, a RFIDreader 1504 may issue commands (see transaction 1 in FIG. 15) toretrieve (see transaction 2 in FIG. 15) the values contained in digitalshift register 1502 via conventional means, and use that retrievedinformation to evaluate the environment to which tag 1500 is exposed.The evaluation could be as simple as referencing fixed data in memorythat has already been stored and calibrated, or as complex as a softwareapplication running on the reader or its connected systems forperforming interpretive evaluations.

Likewise, consider a tag 1500 containing a field strength (i.e.,received power) detector wherein the method of operation of the systemcontaining the RFID tag 1500 calls for field strength detector toselectively perform a sweep function and developing the quantizeddigital representation of the current via the method discussed earlier.As illustrated in FIG. 15, counter 818 will contain the digitalrepresentation developed by our field strength detector 610 of the RFsignal induced current, and may be used either internally by the RFIDtag 1500, or read and used by the reader 1504, to discern relativeenvironmental information to which the RFID tag is exposed. For example,reader 1504 may issue a command to the RFID tag 1500 to activate tuner606 and/or detector 610 and, subsequent to the respective operations oftuner 606 and/or detector 310, receive the digital representations ofeither the matched impedance or the maximum current developed duringthose operations. Once again, this digital value of the field strengthstored in the counter 818 may be used either internally by the RFID tag1500, or read and used by the reader 1504, to discern relativeenvironmental information to which the RFID tag 1500 is exposed. Forexample, tag 1500 may contain a calibrated look-up-table within theclock and control block 824 which may be accessed to determine therelevant environmental information. Likewise, a RFID reader may issuecommands to retrieve the values contained in digital shift register1502, and use that retrieved information to evaluate the environment towhich tag 1500 is exposed. The evaluation could be as simple asreferencing fixed data in memory that has already been stored andcalibrated, or as complex as a software application running on thereader or its connected systems for performing interpretive evaluations.Thus, the combining of the technologies enables a user to sense theenvironment to which a tag 1500 is exposed as well as sense changes tothat same environment.

Some environmental factors can change the effective impedance of theRFID antenna. Thus, it is possible to dynamically retune the tankcircuit 604 or other like impedance to compensate for theenvironmentally-induced change in impedance by systematically changingthe digital tuning parameters of tank circuit 604. By characterizing theantenna impedance as a function of various factors, one can develop anestimate of the relative change in the environmental factor as afunction of the relative change in the digital tuning parameters of thetank circuit 604.

As can be seen in Table 1, above, it is possible to develop, a priori, areference table storing information relating to a plurality ofenvironmental reference conditions. Thereafter, in carefully controlledconditions wherein one and only one environmental condition of interestis varied (see, FIG. 16), an operational tag 1500 is exposed to each ofthe stored reference conditions (step 1602) and allowed to complete thetank tuning process. (recursive steps 1606 and 1608. After tuning hasstabilized, the RFID tag 1500 can be interrogated (step 1610), and thefinal value in the shift register 1502 retrieved (step 1610). This valueis then stored in the reference table in association with the respectiveenvironmental condition (step 1612). The resulting table might look likethis:

TABLE 2 Tuning Parameters vs. Frequency 860 870 880 890 900 910 920 930MHz MHz MHz MHz MHz MHz MHz MHz In Air 25 21 16 12 8 4 0 0* On Metal 3127 22 17 12 8 3 0  On Water 20 19 12 12 4 0  0* 0* On Glass 21 17 12 8 4 0*  0* 0* On Acrylic 23 19 14 10 6 2  0* 0* 0* indicates that a lowercode was needed but not available; 0 is a valid code.

In contrast to prior art systems in which the antenna impedance must beestimated indirectly, e.g., using the relative strength of the analogsignal returned by a prior art tag 1500 in response to interrogation bythe reader 1504, methods of the present disclosure employ the on-chipre-tuning capability of our tag 1500 to return a digital value whichmore directly indicates the effective antenna impedance. Using areference table having a sufficiently fine resolution, it is possible todetect even modest changes in the relevant environmental conditions. Itwill be readily realized by practitioners in this art that, in generalapplications, environment conditions typically do not change in an idealmanner, and, more typically, changes in one condition are typicallyaccompanied by changes in at least one other condition. Thus, antennadesign will be important depending on the application of interest.

One possible approach mounts the antenna on a substrate that tends toamplify the environmental condition of interest, e.g., temperature.

Shown in FIGS. 17A and 17B is an RF sensing system 1700 constructed inaccordance with one embodiment of embodiments of the present disclosure,and specially adapted to facilitate sensing of one or more environmentalconditions in a selected region surrounding the system 1700. In general,the system 1700 comprises: an RF transceiver 1706; a di-pole antenna1708 comprising a pole 1708A and an anti-pole 1708B; and a tail 1710 ofeffective length T, comprising respective transmission line pole 1710Aand transmission line anti-pole 1710B, each of length T/2. Tail 1710includes a sensing portion having the transmission lines and atransporting portion 1712. Transporting portion 1712 may transport adisturbance from a remote location towards the sensing portion. In oneembodiment transport portion 1712 wicks fluids/moisture/wetness from aremote location to be monitored to the sensing portion where theproximity of the sensing portion to the wicked fluids/moisture/wetnessalters the load impedance of the transmission lines. The transportingportion 1712 and sensing portion may entirely overlap in certainembodiments or partially overlap in others. In accordance withembodiments of the present disclosure, the differential transmissionline elements 1710A-1710B are symmetrically coupled to respective poles1708A-1708B at a distance d from the axis of symmetry of the antenna1708 (illustrated as a dotted line extending generally vertically fromthe transceiver 1706). In general, d determines the strength of theinteraction between the transmission line 1710 and the antenna 1708,e.g., increasing d tends to strengthen the interaction. In theequivalent circuit shown in FIG. 17B, the voltage differential betweenthe complementary voltage sources 1708A and 1708B tends to increase as dis increased, and to decrease as d is decreased. Preferably d isoptimized for a given application. However, it will be recognized thatthe sensitivity of the antenna may be degraded as a function of d if aload, either resistive or capacitive, is imposed on the tail 1710.

In operation, the tail 1710 uses the transmission line poles 1710A-1710Bto move the impedance at the tip of the tail 1710 to the antenna 1708,thus directly affecting the impedance of the antenna 1708. Preferably,the transceiver 1706 incorporates our tuning circuit 606 so as to detectany resulting change in antenna impedance and to quantize that changefor recovery, e.g., using the method we have described above withreference to FIG. 16.

FIG. 18 illustrates one possible embodiment of the system 1800 in whichthe antenna poles 1708A-1708B are instantiated as a patch antenna(illustrated in light grey), with the antenna pole 1708A connected toone output of transceiver 1706, and the other output of transceiver 1706connected to the antenna anti-pole 1708B. A ground plane 1712A(illustrated in a darker shade of grey than the patch antenna 1708) isdisposed substantially parallel to both the antenna poles 1708A-1708Band a ground plane 1712B disposed substantially parallel to thetransmission line poles 1710A-1710B. As is known, the ground planes 1712are separated from the poles by a dielectric substrate (not shown),e.g., conventional flex material or the like. If the dielectric layerbetween the antenna poles 1708 and ground plane 1712A is of a differentthickness than the layer between the transmission line poles 1710 andthe ground plane 1712B, the ground plane 1712B may be disconnected fromthe ground plane 1712A and allowed to float. In general, this embodimentoperates on the same principles as described above with reference toFIGS. 17A and 17B.

Shown in FIG. 19 is one possible flow for a sensing system 1700 usingthe antenna 114. As has been explained above with reference to FIG. 16,operations 1900 begins with the sensor being first calibrated (step 1902to detect the presence of varying levels of a particular substance. Forthe purposes of this discussion, we mean the term substance to mean anyphysical material, whether liquid, particulate or solid, that is:detectable by the sensor; and to which the sensor demonstrably responds.By detectable, we mean that, with respect to the resonant frequency ofthe antenna in the absence of the substance, the presence of thesubstance in at least some non-trivial amount results in a shift in theresonant frequency of the antenna, thereby resulting in a concomitantadjustment in the value stored in the shift register 1502; and bydemonstrably responds we mean that the value stored in the shiftregister 1502 varies as a function of the level the substance relativeto the tip of the tail 1710 of the antenna 1700. Once calibrated, thesensor can be installed in a structure (step 1904), wherein thestructure can be open, closed or any condition in between. The structurecan then be exposed to the substance (step 1906), wherein the means ofexposure can be any form appropriate for both the structure and thesubstance, e.g., sprayed in aerosol, foam or dust form, immersed inwhole or in part in a liquid, or other known forms. Following a periodof time deemed appropriate for the form of exposure, the sensor isinterrogated (step 1908) and the then-current value stored in the shiftregister 1502 retrieved. By correlating this value with the table ofcalibration data gathered in step 1902, the presence or absence of thesubstance can be detected (step 1910).

In one embodiment, the table of calibration data can be stored in thesensor and selectively provided to the reader during interrogation toretrieve the current value. Alternatively, the table can be stored in,e.g., the reader and selectively accessed once the current value hasbeen retrieved. As will be clear, other embodiments are possible,including storing the table in a separate computing facility adapted toselectively perform the detection lookup when a new current value hasbeen retrieved.

FIG. 20 is a block diagram of a RFID wireless solution provided byembodiments of the present disclosure. Integrated circuit (IC) 2000comprises a memory module 2002, a wireless communication engine 2004,and a sensor engine 2006 which includes an antenna 2008. IC 2000 iscapable of sensing a change in the environmental perimeters proximate toIC 2000 via impedance changes associated with antenna 2008. In otherembodiments, a proximity sensor may be employed to determine theproximity of IC 2000 to a given location or RFID reader by tuning theantenna 2008 and an associated tunable impedance. Memory module 2002 iscoupled with both the wireless communication engine 2004 and sensorengine 2006. Memory module 2002 is capable of storing information anddata gathered by sensor engine 2006 and communicated via wirelesscommunication engine 2004. Further, wireless communication engine 2004and sensor engine 2006 may be fully programmable via wireless methods.Passive RFID sensors of FIG. 20 may be deployed as an array of smartsensors or agents to collect data that may be sent back to a centralprocessing unit.

FIG. 21 is a block diagram of one arrangement of smart sensors and adata processing unit 2102 in accordance with embodiments of the presentdisclosure. Here a series of passive RFID sensors 2000A-N are deployedwherein each sensor may have a unique identification number storedwithin the memory module and communicated via the internal wirelesscommunications engine 2004 to a data processing unit. Interrogator (RFIDreader) 2104 interacts with passive RFID sensors 2000A-N. Interrogator2104 may then communicate with a data processing unit 2102. Thus, thepassive RFID sensor array 2106 may allow information to be sensed andcommunicated via RFID reader 2104, wherein this information may bepre-processed at the passive RFID sensor, or remotely processed at theRFID reader 2104 or data processing unit 2102 depending on the systemneeds.

Embodiments of the present disclosure realize an advantage over priorsystems, in that not all sensing requires high precision sensors whichare both expensive and consume relatively large amounts of power. Thesensors provided by embodiments of the present disclosure are relativemeasurements and post processing of collected measurements yields senseinformation. Calibration may be done during manufacturing at the waferor die level or when the assembled sensors are deployed in the fieldwherein this calibration information may be stored in the memory module2002. This information may be retrieved at any time for baselinecalculations. From relative changes, accurate information may then bederived from remote data processing provided by data processing unit2102. Calibration may involve retrieving sensing measurements frommemory module 2002 or current measurements directly form sensor engine2006. The use of this information then allows accurate data associatedwith environmental conditions to be determined. In one example, RFIDsensor array 2106 of FIG. 21 may include temperature sensors. Whereineach passive RFID sensor 2000A-N is an independent sensor and may sensea current condition at time zero that is stored to memory module 2002 orsent to data processing unit 2102. This measurement may be repeated atTime 1. Wherein this data is either stored or transmitted. Dataprocessing unit 2102 may perform more complex calculations. For example,if the temperature is known at Time 0, the sensor information collectedat Time 1, when communicated may be processed using informationassociated with the measurements and known temperature at Time 0 inorder to determine or approximate an actual temperature. This mayinvolve a lookup in a characterized data table or computations based onmathematical models of the calibration of the sensors to determine orapproximate the actual temperature.

Another embodiment can sense the level of wetness or humidity proximateto the sensor engine. In either case, temperature or moisture, raw datamay be collected from passive RFID sensors via the RFID reader forprocessing to be performed by data processing unit 2102 where thecomputation to determine a humidity or temperature measurement.

FIG. 22 provides an illustration of an antenna arrangement in accordancewith embodiments of the present disclosure. In this antenna arrangement2200 the antenna comprises a first antenna wing 2202, a second antennawing 2204, and a tail 2206 coupled to IC 2000.

Tail 2206 includes a sensing portion 2208 having the transmission linesand a transporting portion 2210. Transporting portion 2210 may transporta disturbance from a remote location towards the sensing portion. In oneembodiment transport portion 2210 wicks fluids/moisture/wetness from aremote location to be monitored to the sensing portion where theproximity of the sensing portion to the wicked fluids/moisture/wetnessalters the load impedance of the transmission lines. The transportingportion 2210 and sensing portion 2208 may entirely overlap in certainembodiments or partially overlap in others.

IC 2000 may optimize the impedance match between the IC 2000 and antenna2200 and tail 2206. This can be accomplished by adding shunt capacitors,variable inductors or variable impedances across the input terminals ofIC 2000. As a result, the input impedance of the integrated circuit canbe varied, in one embodiment, between 2.4 minus J 67.6 to 0.92 minus J41.5 ohms. An antenna such as that provided in FIG. 22 may be designedto operate within these impedance values.

In one embodiment, this may provide an RF sensitivity of approximately−10.5 DbM. The antenna provided in FIG. 22 may be optimized to provide aconjugate match in one embodiment at about 960 megahertz. This allowsthe integrated circuit to optimize and match by selecting the best selftuning value over the remaining portion of the frequency band. Theoperational bandwidth is proportional to the RFID tag thickness.

The antennas provided by embodiments of the present disclosure may befabricated in one embodiment using flex PCB materials. Electricalconnections between the bumps of the integrated circuit and the antennaallow the antenna and integrated circuit to be electrically coupled.

FIG. 23 is a view of an RFID moisture or humidity sensing tag 2300 inaccordance with an embodiment of the present disclosure. Moisture orhumidity sensing 2300 is a passive RFID tag, which includes a sensor,the sensor having a variable sensor impedance, and IC 2000. The sensorimpedance varies as the coupling of interdigitated capacitor 2304responds to environmental changes. These changes may be wicked to theinterdigitated capacitor 2304 by tail 2308 which may be entirelycomposed of a material that transports fluids/moisture/wetness to theinterdigitated capacitor 2304 by capillary action. More generally tail2308 transports environmental disturbances to the interdigitatedcapacitor 2304. In one embodiment, interdigitated capacitor 2304 islocated proximate to a film 2306 applied above interdigitated capacitor2304. Film 2306 may be a material having an affinity for water (i.e.moisture or humidity) or other fluids. These fluids may include CO, CO2,Arsenic, H2S or other known toxins or gases of interest. When film 2306absorbs a fluid such as those described previously, the dielectricconstant proximate to the interdigitated capacitor 2304 changes causingan impedance change. The impedance of the interdigitated capacitor 2304sensed by the processing module coupled to the sensor then produces anoutput, a sensor code, representative of the absorbed material withinfilm 2306. This data may be stored within a memory circuit of IC 2000 ortransmitted to an external reader by the wireless communication moduleof IC 2000.

FIG. 24 is a view of a folded RFID tag 2400, including antenna 2402comprising a radiating element, the radiating element comprising a firstwing 2402A and a second wing, the second wing divided into a proximalsection 2402B and a distal section 2402C, the distal section 2402Cfolded onto the proximal section 2402B, and the first wing 2402A foldedonto the folded second wing, the distal section 2402C of the second wingcapacitively couples to the proximal section 2402B and the first wing2402A. Tail 2410 includes a sensing portion 2414 having the transmissionlines and a transporting portion 2412. Transporting portion 2412 maytransport a disturbance from a remote location towards the sensingportion. In one embodiment transport portion 2412 wicksfluids/moisture/wetness from a remote location to be monitored to thesensing portion where the proximity of the sensing portion to the wickedfluids/moisture/wetness alters the load impedance of the transmissionlines. The transporting portion 2412 and sensing portion 2410 mayentirely overlap in certain embodiments or partially overlap in others.These sections are folded about a PCB core.

FIGS. 25A-C are block diagrams of arrangements of a self-tuning engineto support the reporting of several stimuli with multiple passive RFIDsensors using an antenna impedance sensing mechanism in accordance withembodiments of the present disclosure. Module 2500 includes antennaports 2502A-N, self-tuning engines 2504A-N, processing unit 2506,reference input module 2508 and power harvesting module 2510. A numberof antenna ports 2502A-N passively sense stimuli through changingantenna inductance as previously discussed. The self-tuning engines2504A-N adjusts a variable capacitance 2512A-N in response to theinductance sensed as ADC 2514A-N wherein decision module 2516A-N directsfeedback to adjust the value of variable capacitance 2512A-N and producea code reported to processing unit 2506. This sensor code reflects thesensed stimuli relative to the antenna inductor 2518A-N. The stimulisensed may be any combination of stimuli sensed by the changinginductance of the antenna (i.e. pressure, moisture, proximity etc.)Processing unit 2506 is coupled to the self-tuning engines 2504A-N andother potential reference inputs such as those provided by referenceblock 2520. Reference block 2520 allows the processing unit tocompensate for external elements sensitive to external stimulus with aninput to processing unit 2506. One such example may be where an externalelement is sensitive to a condition such as temperature, in this examplereference block 2520 provides a reference signal 2522 for the processingunit 2506. The block as a whole may be powered by a power harvestingengine 2510 to supply on-chip power needs.

Embodiments of the present disclosure encompass the ability for thepassive RFID tag to (or based on the data supplied by the RFID tag) tomake decisions based on multiple sensory inputs. Implemented in anon-chip signal processing circuit, single self tuning engine 2504A-Nautomatically adjusts the input impedance to optimally tune the RFID tagevery time it is accessed.

RFID tags based on conventional chips can be detuned by a variety ofexternal factors, most commonly by proximity to liquids or metals. Suchfactors can change the impedance characteristics of a tag's antenna.When the RFID tag chip has a fixed impedance, a mismatch between thechip and the antenna results, reducing the RFID tag's performance. Selftuning engine 2504A-N maintains the chip-antenna match as conditionschange, resulting in more consistent RFID tag performance.

Reference signal 2522 is basically a reference voltage that is generatedby an external sensing mechanism. In combination with one or more of thesingle self tuning engine 2504A-N, various decisions (e.g. co-dependentdecisions) and sensing can be made based on various parameters collectedfrom these multiple ports. A device can be interfaced to providereference signal 2522. Examples of such devices include an accurateresistor (e.g. 1% resistor) between used to calibrate the variouscircuitry or sensors. The 1% resistor value can be digitized tocalibrate temperature or pressure measurement. Other examples include: Aphotodiode to sense light; A pin diode; A remote temperature sensor; AnLED (Light Emitting Diode); An infrared (I/R) sensor; and Basic I/O,ADC, DAC to input/output data from/to the sensor chip

It may be desired to eliminate process variations or temperaturevariations from a sensing measurement (e.g. gas sensing application).

FIG. 25B illustrates yet another embodiment with two external sensingports providing reference signals 2522A and 2522N. As discussed invarious parts of this disclosure, the sense material can cause a changein resistance due to an environmental variable that is to be sensed andthus would affect the variation sensing leg differently than its effecton the rest of the resistors. All other environmental variables wouldaffect the four balanced resistors equally and as such would becalibrated out and would not be sensed isolating the effect to theenvironmental variable affecting the variation sensing leg via theapplied sense material.

One of the differences of sensing using an antenna port vs. an externalelement port is the fact that the sensing via the antenna port uses ACpower generated by the application of a CW (continuous wave). Thesensing on an external element port uses DC power that is generated viathe power harvesting engine (using one or more charge pumps) asexplained below. Given the fact that a charge pump efficiency of about20% results in approximately five times increase in power consumption bysensing using the external element port vs. the antenna port (sensingusing high frequency rather than DC).

Power harvesting engine 2510 generates DC power using one or more chargepumps. The charge pump is included in for example regulator 608 of FIG.6, regulator 702 of FIG. 7, regulator 608 of FIG. 8, regulator 316 ofFIG. 3 and regulator 316 of FIG. 5. Given the fact that a charge pumpefficiency can greatly vary. The efficiency of a charge pump equalspower delivered to its output (i.e. to the rest of the circuit it is toprovide a supply voltage and current) divided by power consumed at itsinput. A charge pump, as known in the art, is continuously switching andwhose voltage waveforms vary with time, so in general, efficiency can bemeasured as the ratio of the average power at input and output, asopposed to the ratio of instantaneous powers. A simple low efficiencycharge pump can be designed to have a very quick startup time but willresult in a great loss of power and thus be unable to operate a largeamount of circuitry or sustain the operation of a circuit over a longerperiod of time compared to a higher efficiency charge pump. On the otherhand, a higher efficiency charge pump in general will take a longerperiod of time to startup, but will be able to operate more circuitrygiven the same input power and would be able to sustain operation over alonger period of time that its low efficiency counterpart.

In order to achieve quick startup and efficient operation, a set ofcharge pumps may be used having different efficiencies. A first chargepump may be used to initially energize the RFID sensor. Once essentialcircuitry is operational, additional more efficient charge pumps may beused to energize the sensor and the remaining circuitry. This allows fora shorter time requirement to initialize the RFID sensor. Longer termoperation of the RFID sensor may then be switched to the more efficientcharge pump.

The charge pump(s) harvest power from the ports 2502A-N and thensupplies DC power to the rest of the circuitry. Another embodimentincludes the use of two or more charge pumps one corresponding to anindividual port and then combining the currents from both in order toproduce the DC supply voltage for the RFID Sensor. In yet anotherembodiment, two charge pumps can be used for a single self tuning engine2504A-N regardless of whether the RFID tag a single or multiple singleself tuning engine 2504A-N/Sensing antenna 2518 arrangements.

Two or more charge pumps may be coupled to an individual antenna. Onecharge pump to turn on the single self tuning engine 2504A-N quickly,and is thus optimized for low turn-on power which sacrifices efficiency.The second charge pump has a higher turn on power threshold but has amuch higher efficiency. Both charge pumps may operate in parallel butresults in a much faster turn-on time for the RFID tag.

Once sufficient stable power is available, power harvesting engine 2510will produce a PowerOK signal to initiate system as seen in FIG. 4. Avariable resistor can be provided in parallel with an inductor, so as toautomatically vary this resistance to control the gain of the tankcircuit within the power harvesting engine 2510.

The operation of the self-tuning engine in response to the PowerOKsignal is illustrated in the description of FIG. 3, FIG. 4 and FIG. 5.

Another aspect of this disclosure is extending a mode that would allowfor self-operation without the need for a reader but only a continuouswave (CW) source for power. In this self-operation mode, sensor valuescould be self-written in a user defined circular buffer in the memory(or other types of memories).

This mode would be entered with a header length in excess of specifiedperiod of time. For a typical transaction, the part would power up inthe typical ready state to accept commands from a reader and respondlike a traditional tag. Once a sufficient amount of time had passed anda command was not received, the part would enter into the self-operationmode of data logging. Some control and status registers could be presetby the user to configure this mode that could include:

d. data buffer size (1-x words in user bank)

e. data buffer pointer/index

f. Sensor to log (Self-tuning engine and/or Temp)

g. Max/Min threshold value

h. Max/Min Threshold exceeded count

Every time the RFID sensor entered this logging mode the RFID sensorwould measure/log the data, auto increment the pointer/index to the nextword in the buffer and then hold in an idle state. If thresholds wereemployed, a count could simply be maintained for any measurementsover/under the threshold. This is useful in applications like cold chainmanagement of produce or pharmaceuticals where the customer only caresif a perishable product has fallen outside of a specified temperaturewindow.

The primary benefit of this self-operation mode would be the low cost CWsources that could be utilized instead of full reader to create awireless logging system. The CW source would essentially just be tied toa timer that would control when and how long it was turned on. The timeron the CW source would set the data logging interval with one sampletaken every time the CW source was turned on.

The CW sources would be used throughout the system to maintain datacollection operation for the RFID tags and then readers would only beneeded at the endpoints of the system to gather the data logged. For acold chain application, the low cost CW sources could be placed in therefrigeration trucks or warehouses and then the customer would verifythe product condition with a reader when it arrived at market. This isan economically viable idea since the infrastructure required to simplygenerate CW source would be much less than implementing a fullfunctionality reader and communication capabilities.

Yet another aspect of this disclosure is the IC harvesting power from aCW source that is a different bandwidth than the UHF bandwidth for RFIDreaders in the United States (902-928 MHz). But rather using a 2ndSelf-tuning engine engine/antenna/sensor port in that is tuned for otherfrequency bands such as an ISM band source.

Another aspect of this disclosure is the integration of an antenna, selftuning engine and processing circuitry as part of a silicon wafer or alarge IC (e.g. Microprocessor die, FPGA die). Such integration wouldenable a variety of applications that are currently not possible withoutpowering an IC (e.g. Micro-processor or FPGA). For example, the abilityto embed a serial number on in a Magnus register and to use an RFIDreader to inventory the ICs. Such devices are very expensive andvaluable and the ability to inventory each IC individually would providegreat economic benefit such as saving time, fraud control and inventorycontrol.

Additionally, having the sensor functionality can alert manufacturers,vendors, distributors and customers pre-production, post-production,pre-shipping, post-shipping and in the field to any exposure toenvironmental variables that are critical to the economic value andoperation of the IC. Such harmful exposure is moisture, for example. Anyof the sensory applications mentioned earlier in this application ispossible.

Additional sensory applications are the detection of exposure to fluids(which includes gases, such as Oxygen for example). The locations ofembodiments of the present disclosure do not have to be part of the ICdesign process but rather as part of the scribe area.

FIG. 26 is a block diagram of a self-tuning engine in accordance withembodiments of the present disclosure. Self-tuning engine 2600 includesan antennae 2602, a variable capacitance or varactor module 2604, aclock acquisition and data conversion module 2606, a monitoring module2608, a decision module 2610, processing module 2614, and a clock module2612.

Varactors are basically voltage-controlled capacitors. Varactors areimplemented in various forms, for example as discrete components, inintegrated circuits, in MEMS (micro-electro-mechanical systems).Varactors are widely used in RF circuits as tuning elements.

FIG. 27 illustrates an embodiment of the self-tuning engine provided byembodiments of the present disclosure along with the varactors that aredriven by the tuning circuitry (referred to as self tuning engine). Thevaractors in this embodiment are enhancement MOS varactors. In oneembodiment, the engine generates 5 bits of sensor code (also referred toas MMS code) that are then converted to 16 bits (i.e. n=16) ofthermometer codes. Each bit of the thermometer code drives one varactorunit. In this embodiment, there are a total of 16 varactor units (eachunit is a varactor on its own). Each code can be either VDDA (a highvoltage) or VSSA (a low voltage signal). The antenna ports; ANTP andANTN, are set at a voltage value of VDDA/2.0 under normal operation.Looking at this from the varactor perspective, the Gate of each of the16 varactor units will always be at VDDA/2.0V with respect to Bulk,while the S/D, (Source/Drain), connection of each of the 16 varactorunits will be set to VDDA or 0V with respect to Bulk, depending on thesensor code generated. Hence, each of the 16 varactor units will be setto either its min capacitance or max capacitance value. The totalcapacitance of the varactor structure is the sum of these min/maxvalues. This implementation is referred to here as a digitalimplementation of an embodiment of the self-tuning engine provided byembodiments of the present disclosure.

One embodiment of the present disclosure uses non-equal capacitors inthe self-tuning engine with no simple ratio metric relationship (e.g.integer multiple or ratio of integers) to implement dithering.

Returning to FIG. 26, the clock acquisition and data conversion module2606 will sense a voltage associated with the variable capacitance orvaractor 2604 that may change as a function of antennae impedancewherein the impedance is changed based on environmental stimulus orother like conditions. Monitoring module 2608 may monitor phase andamplitude or other qualities associated with the data collected by clockand data conversion module 2606. This information is then provided toprocessing module 2614 which in conjunction with decision module 2610may place capacitors 2616 A through N in service within the variablecapacitance or varactor 2604 in order to maximize power transfer orother like considerations with antennae 2602. The manipulation of thevaractor 2604 will relate to a sensor code as discussed previously orother like signal. Clock 2612 provides a clock input to the variousmodules within Engine 2600 such that the data acquisition and theactions of the various processing modules may be coordinated.

Embodiments of the present disclosure may provide a passive RFID sensor(IC chip, antenna, and package) such that once an event of interest hasoccurred, the structure of the antenna and package may change itscharacteristics in an irreversible manner. FIG. 28 provides a flow chartof one such embodiment. In Block 2802, a passive RFID sensor, such as anantenna may be inlaid within the structure wherein a physicalcharacteristic of the antenna and/or the sensor, such as impedance, maybe altered when exposed to a sudden force. For example, an antenna maybe wrapped around a glass or other structure. The original uniqueimpedance value may be recorded and stored for comparison in block 2804.In block 2806, the impedance value may be read on an ongoing basiswherein when the impedance value or a code associated with the impedancevalue changes, that change signals that the event of interest may haveoccurred. Such an event may be when an object on which the passivesensor is mounted has been dropped.

In an embodiment, step 2806 of FIG. 28 can be altered so rather thansensing on an ongoing basis, the impedance is read at a later time thatis offset from the event that caused the original unique impedance tochange value. The sensing in step 2808 thus indicates that theparticular event occurred that changed the original unique impedancebecause the new code read is different than the original code recordedin step 2804. The recording can be locally on the tag itself via anon-volatile memory or in a database remote from the tag as isassociated with the unique identification number of the tag. In any ofthe cases, the magnitude of the impedance change, results in a differentcode change and thus is used to also detect the magnitude or amount ofexposure to an event or and environmental change.

FIGS. 29A and 29B illustrate one passive RFID sensor in accordance withan embodiment of the present disclosure. FIGS. 29A and 29B provide anembodiment of an RFID tag with a particular antenna structure being adual-mode dipole design designed to perform directly on the skin. TheRFID moisture sensor may be optimized to achieve roughly equalperformance whether the antenna is tight against the skin or spacedabout 1 cm from the skin. This results in a balanced design to providethe same read range whether the band is tight or loose on the wrist.FIGS. 29A and 29B show a WristTag Antenna constructed with an antennathat is made of ½ oz copper (Cu) on 10 mil PET with no overlay. The IC2000 may be flip-mounted directly to the Cu using anisotropic conductiveadhesive (ACP).

FIGS. 30-38 are graphs of several measurements taken using two differentsamples (sample 1 and sample 2) while on a human wrist and on a 2.5 inchdiameter water bottle in different configurations. The placement on the2.5″ bottle is used to simulate placement on a human wrist. The resultspresented show that the results are similar.

In the FIGs., the sensitivity (measured as explained previously), isplotted (red solid line, in dB) and the sensor code from the impedancematching engine is also plotted (discrete circles). Noted on the figuresare range estimates for max FCC power and linear polarization. FIG. 30is a graph of several measurements taken using a WristTag placed tightlyon a human wrist. FIG. 31 is a graph of several measurements taken usinga WristTag placed loosely on human wrist (about 10 mm air gap). FIG. 32is a graph of several measurements taken using a WristTag placed on 2.5″diameter water bottle (results very similar to on-wrist measurements).FIG. 33 is a graph of several measurements taken using a WristTag(Sample 2) placed on 2.5″ diameter water bottle with tag in alternatepositions: facing down and facing to side (Bottle seems to focus the RFpower, the WristTag works better facing away from the reader). FIG. 34is a graph of several measurements taken using a WristTag off bottle(with 10 MIL PET Spacer inserted between bottle and tag). FIG. 35 is agraph of several measurements taken using a WristTag off bottle (with3.5 mm air gap between bottle and tag)—these show best performancenumbers. FIG. 36 is a graph of several measurements taken using aWristTag off bottle (with 5.5 mm air gap between bottle and tag). Theresults show a small range reduction compared to the 3.5 mm air gapresults. FIG. 37 is a graph of several measurements taken using aWristTag off bottle (with 9 mm air gap between bottle and tag)—From FIG.37 one can see that the range is similar to 5.5 mm air gap of FIG. 36.FIG. 38 is a graph of several measurements taken using a WristTag offbottle (with 14 mm air gap between bottle and tag)—FIG. 38 shows aslight range reduction compared to the 9 mm air gap measurements. Theseresults demonstrate that the WristTag dual-mode antenna achievesbalanced sensitivities across a range of wristband positions.

As can be seen from the sensor code results, the sensor codes from theSelf tuning engine are in-range for all configurations (no codesaturation) which leaves sufficient codes for adapting to environmentalvariations. The off-wrist read range is improved vs. the on-wrist readrange. The WristTag of FIGS. 29A and 29B operates both tight on thewrist and loose on the wrist. Since the antenna is in close proximity tothe body at all times, the sensor code tends to be the same in allpositions which leaves plenty of codes to operate the tag as a sensor aswell.

By inducing a second mode of operation for tight-on-wrist positioning,the tag sensitivity is maintained while also inducing a large change insensor code.

FIG. 39 is a graph of the WristTag capacitance variation vs. air gap.The capacitance value in Self tuning engine is set by the generatedcode. The graph in FIG. 39 shows that the required tuning capacitance isalmost identical when the tag is 5 or 10 mm from the body, but there isa jump in required capacitance for on-the-wrist operation.

FIGS. 40, 41 and 42 show plots of the internal currents in the antennawith 10 mm gap, 5 mm gap and no gap between antenna and wrist. As can beseen from FIG. 40, the flip in the current direction for tight-on-wristoperation compared to an off-wrist operation (with a gap) as seen inFIG. 40 and FIG. 41 represent a second mode of operation for theantenna. The second mode of operation requires a substantially differenttuning capacitance from the die, and this leads directly to asubstantially different sensor code. The significant change in thesensor code for on-the-wrist vs. off-the-wrist operation enables the tagto perform a sensor function. For example, the sensor code can be usedto determine the proximity of the tag to skin.

Another embodiment of a passive RFID moisture tag/sensor is a diapertag. The diaper tag is a variation of the WristTag discussed in theprevious section that works for a diaper application.

The differences between the diaper tag and the WristTag involveoptimizing basic dimensions as required since the diaper tag lays flatand the WristTag wraps around (flat vs. curved), and, improving theperformance with three added features. These features include: boxedextensions on the ends, slits in the middle, and a pad of metal insidethe tuning inductor. The dual-mode behavior is explained in the previoussection. The same physical mechanism occurs for the diaper tag.

FIG. 43 is a diagram of an embodiment of a diaper tag/sensor inaccordance with an embodiment of the present disclosure. In theembodiment, the antenna is a dipole antenna where the radiating elementsare the metal sheets extending in two directions and are looped aroundfor various reasons (also referred to as a folded dipole). The dipoleuses a standard T-match configuration. Inside of the T-match is acapacitive pad used for tuning purposes. The following measurements onthe tag of FIG. 43 were taken on a 1 gallon bottle and all themeasurements were taken broadside (i.e. the reader and tag antennas areparallel with the centers lined up). The one gallon bottle is similar toa one gallon milk jug with four corners. Water is added to the diaper atthe upper center point and the lower center point. The water was addedin 30 ml increments centered on the tag while the diaper stays in placeand held down by 3 rubber bands. The “corner” of the water bottlecreates a compression line which creates a pinching effect. This causesall absorption on the front side (where the tag is affixed) whichbecomes highly saturated as water is added. The results show excellentbehavior across the 902-928 MHz frequency range and exhibit very largecode shifts making the addition of water very detectable by RFIDtag/sensor.

FIG. 44 is a plot of average sensitivity as water is added for variousfrequency ranges. FIG. 45 is a plot of average sensor code as water isadded for various frequency ranges. As can be seen, there is a largecode shift for all the frequency ranges from the dry (0 ml) to the wetconditions. FIG. 46 is a plot of showing the code shift magnitude from adry condition vs. amount of water added for the various frequencyranges.

As with the first experiment, FIGS. 47, 48 and 49 show results ofexcellent behavior across the 902-928 MHz frequency range and exhibitvery large code shifts making the addition of water very detectable byRFID tag/sensor.

FIG. 47 is a plot of average sensitivity as water is added for variousfrequency ranges. FIG. 48 is a plot of average sensor code as water isadded for various frequency ranges. As can be seen, there is a largecode shift for all the frequency ranges from the dry (0 ml) to the wetconditions. FIG. 49 is a plot of showing the code shift magnitude from adry condition vs. amount of water added for the various frequencyranges.

FIGS. 50, 51 and 52 show results where the tag was mounted with thecenter 120 mm from the edge of the Super-Absorbent Polymer (SAP) filler.Water is added to the center of the diaper. The water was added in 30 mlincrements centered on the diaper that is, offset to the tag. In thisexperiment, the diaper is removed to add water to avoid the “corners”of/the jug from limiting absorption. The bulge that forms is at a 45degree angle to the antenna and causes a small lensing effect near 180ml.

As with the previous experiments, the results show excellent behavioracross the 902-928 MHz frequency range and exhibit very large codeshifts making the addition of water very detectable by RFID tag/sensor.Additionally, the sensitivity for the high water loadings is improved.FIG. 50 is a plot of average sensitivity as water is added for variousfrequency ranges. FIG. 51 is a plot of average sensor code as water isadded for various frequency ranges. As can be seen, there is a largecode shift for all the frequency ranges from the dry (0 ml) to the wetconditions. Additionally, there is additional shift at the high waterlevel (near 180 ml). FIG. 52 is a plot of showing the code shiftmagnitude from a dry condition vs. amount of water added for the variousfrequency ranges. There is additional shift at the high water level(near 180 ml).

As can be seen from the data above, the diaper tag exhibits thefollowing behavior. The performance is well centered on the NorthAmerica 902-928 MHz band. Sensor code movement is robust across allmeasurement setups and diaper fill levels. Easy binary decision: dry forcodes<10, wet for codes>10. However, Sensor codes are not giving anindication of “how wet”. Sensitivity steadily degrades as the diaperbecomes more wet. Wetness centered on the tag measures worst-casesensitivity of about 4 dBm. Offsetting the tag from the wetness measuresworst-case sensitivity of about 2 dBm with no penalty in code movement.The offset tag should also produce better sensitivity relative tocentered tag due to the raised position of the offset tag giving betterline-of-sight to the reader.

Expected field behavior using offset tag placement is as follows: Drydiaper—Best-case read range of about 3 feet. Sensor codes<5. Slightlywet diaper—Read range improves to a maximum of about 4 feet. Sensorcode>20. Saturated diaper—Read range degrades to about 2 feet. Sensorcode>20.

FIG. 53 is a plot of the read range for the hand-held reader, best caseread range vs. tag sensitivity. Tag sensitivity is always measuredbecause that is the fundamental measure of performance of the tag. Theread range of the tag depends on the tag sensitivity and several otheruse-dependent variables: reader antenna polarization, reader antennaorientation (if linearly polarized), reader output power, and readerantenna gain. The plot in FIG. 53 calculates the expected best-case readrange given assumptions for these variables as shown in the plot title:reader output power of 27 dBm, reader antenna gain of 5 dBi, andcircular polarization for the reader antenna [so orientation does notmatter]. For this application, these assumptions on the reader enablethe customer to see what kind of read range they can expect since thetag sensitivity is a somewhat abstract concept to them and they do notknow how to convert tag sensitivity to read range. So, the readerconsiderations were: Assumed: 27 dBm output power, 5 dBi gain antenna,circular polarization; 30 dBm output power and linear antenna woulddouble the read range.

Antennas directly on a water/air interface produce asymmetrical fieldpatterns with far more gain into the water than into the air. In effect,the water sucks in the electromagnetic fields. The effect causes theloss of sensitivity as the diaper becomes more saturated. The effect isreduced as the antenna moves away from the water, as in the dry diapercase. The human body will affect the fields differently than bottles ofwater, and this will affect the sensitivity of the tags by an unknownamount.

FIG. 54 is a graph showing Antenna behavior at air/water interface (tophalf labeled 1 is air bottom half labeled 2 is water). Because of thehigh dielectric constant of water, small volumes of water (such as inbottles) tend to resonate and affect results, both in simulation andmeasurement.

It is possible that diffusion in the super-absorbent polymer (SAP) maycause the sensor codes to move over time. Measurements were made to showthe movement of sensor codes over a 1-hour time span. The experimententailed: The diaper is mounted onto the bottle and the diaper ismeasured dry. The specified amount of water is added. The measurementsare repeated at 5 min intervals without disturbing the bottle. Resultsshow that code movement is not sufficient to affect dry vs. wetdecisions. FIGS. 55A and 55B are a set of plots showing sensor codespread vs. time for various frequency ranges and various wetness level(30 ml, 60 ml, 120 ml and 240 ml shown top left, top right, bottom leftand bottom right, respectively).

While all the previous measurements discussed were done with the diapertight against a water jug or bottle, in real life, diapers can developan air gap due to their own weight pulling them away from the skin. Totest for this effect, measurements were repeated with 6 mm of foaminserted between the diaper and the 1 gallon jug of water. Results showthat the “saggy” diaper does not significantly impact the measurementsystem. The sensitivity is slightly improved relative to thetight-on-jug results (experiment #1) and the impact to sensor codes isminor and does not affect the dry vs. wet decisions.

FIG. 56 is a set of plots showing similar to experiments with a 6 mmfoam spacer added between the diaper and the jug.

FIG. 57 is another embodiment of the RFID tag/sensor of the presentdisclosure. The additional changes from the embodiment in FIG. 43 isequalizing the arm lengths on the top and bottom and capping the endextensions. Other embodiments include various changes to end extensionlengths, arm attachment location along with body length, lateral slotpresence, and lateral slot length.

Experiments regarding placement and identical environment for eachmeasurement show significant sensitivity improvement from 1 to 2 dBmacross all wetness levels. It must be noted that both tags provide arobust dry/wet detection ability (threshold, code margin). However, theRFID tag of FIG. 58 alters the load impedance of the transmission linesprovided a higher sensor code for the dry diaper that is tunable up ordown with small impact on sensitivity if necessary. In the embodiment ofFIG. 58, the tab inside the T-match tuning inductor creates anedge-couple capacitor that has little effect in air. With the tag placedon water, the antenna capacitance greatly increases, so a smaller tuninginductor is called for. The edge-coupled capacitor also increases incapacitance tuning out some of the inductance of the tuning inductor andimproving the antenna match.

In the diaper application, the RFID tag is placed so that wetnessinitially occurs past the end of the tag and expands across the tag fromone end with added wetness. If the tuning capacitor is in the center ofthe tag, it may not be needed. The advantage of having a tab is when themoisture hits the diaper directly in the middle of the tab. However, ifthe diaper tag is such that the moisture creeps up on the tag from oneor the other end, then the results show that the tag without the tabwill perform similarly. The moisture hitting the tag directly causes thebiggest electrical effect, the antenna looks more capacitive so the Selftuning engine sensor code needs to move significantly. In order to getsome code sensitivity, the tag codes should not be allowed to saturate(peg at max code) in order to maintain sensitivity and thus read range.

Measurements with the tag mounted off-center from the point where wateris introduced show that the capacitive tab has negligible effect (fromthe edges of the tag). For very wet diapers, the capacitive tab showsbenefits (e.g. for male subjects where the tab is likely to be hitdirectly).

The disclosed RFID tag/sensor are able to detect wetness (moisture)conditions with practical read ranges. In several embodiments, thedesign performs well over an extremely wide range of conditions.Measurements show robust performance of sensor codes to indicate dry vs.wet diapers with a practical read range. Read range is most affected byfully saturated diapers, where the tag at the air/water interfaceexperiences a fundamental reduction in antenna gain. In an embodiment,offset tag placement is recommended to maximize readability of the tag.

In yet another embodiment a universal moisture tag antenna design isshown in FIG. 59. The antenna can be used with, for example, a Selftuning impedance matching engine (MIIC) with a self-tuning engine asdescribed, for example, in association with FIGS. 3, 4, 5, 6, 8, 11, 15and 20. The MIIC chip sensitivity is −16.1 dBm. FIG. 61 is a design ofanother embodiment of a Moisture tag of FIG. 59 antenna (Moisture tag ofFIG. 59—MIIC, 89 mm×24 mm (2136 mm2). The antennas are both foldeddipoles with conventional T-match tuning. The large width helps withmaking the antenna performance fairly consistent with changes to thedielectric constant of the mounting surface.

The moisture tag of FIG. 59 achieves a curling of the current in the endpads by widening the trace before it hits the pad. Measurements weretaken using RFID tags on dry and wet surfaces of various kinds where wetis defined as 1 or 2 sprays from a water sprayer to simulate the wetnesscondition. The various surfaces the tags were attached to for thewet/dry measurements were air (foam), wood, PMMA, PET, PTFE and glass.Additionally, measurements were taken on: drywall that is both dry andthen saturated with water; on metal with a foam spacers of 13 mm, 6 mm,and 2 mm thickness; on a 1-gallon water jug; on a water bottle, tightlyfitted and with 6 mm foam spacer.

The results summarized in the table provided in FIG. 60 were averagedover several materials: Air, wood, PMMA, PET, PTFE, and glass. Drysensitivity difference is 1.31 dBm, basically the same as the differencebetween the published sensitivity difference of the MIIC and Monza RFIDchips of 1.3 dBm.

The moisture tag of FIG. 59 is more robust under water sprays, with nochange in average performance. The moisture tag of FIG. 59 provides morerobust performance over prior art antennas. The increased robustness isdue to the self tuning engine in combination with the new antennadesign. The self tuning engine enables the wet drywall performance toremain flat over the North America band.

The Universal Moisture Tag of FIG. 59 achieves remarkable performancewith a 13 mm spacer. The self tuning engine enables excellentperformance while it is not coded out. Note that the performance for 6mm spacer is affected by MMS interference for codes of 10 and loweruntil it actually codes out at 0. The Self tuning engine enables a 5.9dBm sensitivity advantage for the moisture tag of FIG. 59 for theworst-case tight-on-bottle position for a 2× advantage in read range.The Self tuning engine provides significant performance improvements inseveral areas: Average sensitivity improvement up to 0.5 dBm in waterspray tests; a 2.7 dBm sensitivity advantage for glass sprayed withwater; a 4.6 dBm sensitivity advantage for 13 mm spacing over a groundplane; a 1.2 dBm sensitivity advantage for mounting on a water jug; anda 5.6 dBm sensitivity advantage for mounting tight on a water bottle.

The Moisture tag of FIG. 59 has good code movement in the presence ofwater: the very damp paper towel placed over the tag. The Moisture tagof FIG. 59 is a good moisture tag for situations with large amounts ofwater present (i.e. low sensitivity to water) and the Moisture tag ofFIG. 61 (explained in the next section) as a moisture tag for situationswith small amounts of water present (i.e. high sensitivity to water).

The moisture tag antenna design shown in FIG. 61 may be fabricated using1 oz copper on 5 mil PET with 2 mil adhesive tape coverlay. An exampleof a manufacturing construction would use approximately 9 mm Aluminum onapproximately 2 mil PET with adhesive coverlay. The antenna is a foldeddipole with T-match tuning plus an interdigitated capacitor on top ofthe T-match box. The antenna design is identical to Moisture tag of FIG.59 presented previously except for the configuration of the T-matchtuning inductor and the interdigitated capacitor. Large width helps withmaking the antenna performance fairly consistent with changes to thedielectric constant of the mounting surface. Moisture dependence isinduced by the interdigitated capacitor. The interdigitated capacitoruses wide gaps to increase fringing to help detect moisture and surfacematerials.

Testing using the Moisture tag of FIG. 61 with an MIIC (Self tuningengine) on air (on top of foam), air (on top foam) and repeatedly dryingthe tag, PTFE, PMMA, wood, glass, drywall and PET with wet paper toweldemonstrated that sensor code responds progressively with added water.The sensitivity is on target for −18.2 dBm (−16.1 dBm for the chip minus2.1 antenna gain). Note that some sensitivity is lost due to MMS engineinterference for sensor codes of 10 or less. The tag loses sensitivityfor codes of 10 and lower. The tag in all water conditions has the samesensitivity while the codes are >10. The mechanism that causes the lossof sensitivity is the MMS engine not being finished adaptively tuningbefore the first command arrives due to insufficient power. The adaptiveengine is still tuning, and this corrupts the incoming signal causingloss of communication. This can be avoided by increasing the readerpower so that MMS finishes in time, and this shows up directly as a lossof sensitivity. This only happens for codes of 10 and below and forcodes of 28 and up.

The air measurement when repeated with drying of the interdigitatedcapacitor after each water spray show that sensor code movement isdominated by the capacitor while the antenna is weakly affected bymoisture (the fact that the antenna is weakly affected by addition ofmoisture was shown in the measurements of Moisture tag of FIG. 59).

The Moisture tag of FIG. 59 is insensitive to small and modest amountsof water applied by spraying water onto the tag. The sensitivity towater is increased by adding an interdigitated capacitor to create theUniversal Moisture Tag of FIG. 61, which shows good code movement towater applied by spraying the tag. Note that sensitivity is impactedwhenever the sensor code drops to 10 or less (MMS engine interferencedescribed earlier). For a very wet case, created by fully saturating onelayer of paper towel, the Universal Moisture Tag of FIG. 61 may besensitive with the code pegging low and a big loss of sensitivity. Sincethe Universal Moisture Tag of FIG. 59 is far less sensitive to waterthan the Universal Moisture Tag of FIG. 61, the Universal Moisture Tagof FIG. 59 is highly effective for this case with only a 3 dBm reductionin sensitivity.

The Universal Moisture Tag of FIG. 59 is an excellent foundation formoisture tags in general: Use the tag in its basic form for heavyconcentrations of water. Add an interdigitated capacitor to increasesensitivity to smaller amounts of water. The larger the capacitor, thegreater the sensitivity. The tradeoff is the range of moistureconcentrations to be detected.

In yet another embodiment of the current invention, a moisture tagantenna design is shown in FIGS. 62, 63, 64 and 65. These antennas canbe used with, for example, the IC 2000 and self tuning impedancematching engine (MIIC).

These antenna types may be folded ¼-wavelength patch antenna radiatingfrom one edge. The use is any on-metal application. The prototype of theembodiment is made of 1 oz Cu laminated with transfer tape onto 5 milPET. The spacer is 3M VHB acrylic foam tape 0.4 mm thick. Three layersof foam tape are used to build up the complete spacer ˜1.2 mm thick. TheCu/PET lamination is then cut, IC 2000 is attached, and then foldedaround the spacer. IC 2000 ends up on the inside of the foldedstructure. The production tag may use aluminum on 2 mil PET along withthe 3M VHB spacer.

Other embodiments are constructed by reducing the overhang, where theoverhang is the distance from the metal to the edge of the substrate.The overhang can be increased or decreased without requiring antennare-design.

Yet another embodiment can be constructed by decrease ground planelength (e.g. FIG. 62). The ground plane can be pulled in to improveperformance by a small amount at the expense of increasing performancevariability due to adhesive tape thickness. Typical distance is 2-3 mm.Radiation efficiency is enhanced when the gap between the top metalplate and bottom metal plate is increased. This directly improves tagsensitivity. The spacing between the plates is 1.2 mm for this tag. Ifthe bottom metal plate is cut back by 2-3 mm, then the spacing isincreased by the thickness of the adhesive tape, and let's call that 3mil, or 75 um. The gap is now 1.275 mm, or 6.25% larger. This is not alot, but it is significant. Since the bottom metal plate is capacitivelycoupled to the metal surface on which the tag is attached, they areeffectively shorted together. Now the tag is slightly dependent on thetape thickness.

The embodiments show excellent sensitivity for its size of (−8 dBm),very wide bandwidth, and good response to water that is dependent on theoverhang length.

In yet another embodiment of the current invention, three more moisturetag antenna designs are shown in FIGS. 63, 64 and 65. The antenna can beused with, for example, the IC 2000 with the self tuning impedancematching engine. These antennas have been optimized for sensing moisturein, for example, a stack of wood veneers stacked on top of each other,usually referred to as a pallet.

An example pallet consists of 8′×8′ wood veneers stacked 2 meters high.An example placement of RFID sensors is in the center of the stack. Oneor more tags are inserted into the stack at various positions. Thesepositions can vary in both the X, Y and Z directions. That is, placedbetween different layers at different depths from the edges. For thisembodiment, three such tags are inserted about 1 m from the top andapproximately 20-30 cm from the edge.

Moisture sensing can be achieved with a single tag. In anotherembodiment, the results from multiple sensors may be averaged to producea single sensor code for the pallet. In yet another embodiment, themultiple sensors readings may be weighted in order to provide moreweight to a sensor that is embedded deeper in the pallet oralternatively to provide more weight to a sensor that is at the edge.Such weighting is dependent, for example, on location of where thepallet is to be stored and/or environmental conditions around it.Additionally, such weighting maybe dynamically coprogrammable by theuser in order to change such weighting from one pallet to another. Suchcomputation can be easily programmed in the reader or any other deviceeventually receiving the code.

While various moisture levels can be measured depending on the design ofthe tag, for this particular embodiment moisture content levels from2-10% are to be measured. The system measures the moisture content inabsolute terms with +/−2% accuracy with a read range target that isgreater than 1 m using both fixed and handheld readers.

FIG. 64 shows a conventional compact dipole with T-match optimized to beembedded in wood with dielectric constants ranging from 2 to 5.Conventional design using meandered dipole with T-match and capacitivepad loading at the ends. The dimensions may be optimized to achieve bestperformance when completely buried in moist wood with compositedielectric constants ranging from 2 to 5. Dry wood has a dielectricconstant a little over 2, while wood with moisture content of 10% byweight has a dielectric constant near 5. Performance is defined asmaximum movement in antenna inductance as the capacitance changes from 2to 5 while maintaining antenna gain within ˜2 dB of the ideal dipolegain of 2.2 dBi. Maximum movement in antenna inductance means that theself tuning engine will achieve maximum sensor code movement fordetecting changes in moisture content of the wood. The changing moisturecontent of the wood changes the capacitance of the RFID tag, that theself tuning engine reacts to.

FIG. 63 shows a modification of the antenna of FIG. 64 to add excesscapacitance in the meander routing to increase coupling to the wood forslightly improved sensitivity to moisture level. The design here ismodified to increase the trace width at the bends. Performancedefinition and optimization goals are identical to those used for FIG.64. Since current crowds to the inside of the bends, the extra width hasthe electrical effect of adding capacitance at each bend. Extracapacitance increases coupling to the wood and increases the change inantenna inductance as the dielectric constant of the moist wood changeswith changes in moisture content. The antenna width and length and endpad size are adjusted to maximize performance and antenna gain.

FIG. 65 shows a conventional folded dipole with T-match but withoutmeander routing to potentially reduce sensitivity to knots and otherwood imperfections. This is a conventional dipole design with T-matchand end capacitive pads using folds at the ends instead of meanderrouting to achieve a more compact design. Performance definition andoptimization goals are identical to those used for the antenna of FIG.64. Using the folds instead of meander routing may enable the tag to beless sensitive to tag placement and knots. Testing shows that the tag isactually more sensitive to tag placement and knots.

FIGS. 66, 67 and 68 show the current distribution for FIGS. 64, 63 and65, respectively, at moisture levels of 2% (dielectric constant of 2)and moisture levels of 10% (dielectric constant of 5). The highestcurrent density is shown in red and the lowest current density in darkblue. FIG. 66 shows no change in current density on the bends. FIG. 67clearly shows a change in the current density on the widened bends whichindicates that they are providing a contribution to the Self tuningengine code change due to the capacitance change that results from thechange in the current density. FIG. 68 on the other hand, does not havea meandering trace to reduce antenna length and as a result has a largerdimension. It is more optimized for dealing with wood that have knots.Knots have different dielectric constants than clear wood, so placing anantenna on a knot can skew the sensor code more towards a value for theknot rather than the desired value for clear wood. Low sensitivity toknots is a desirable feature. The meander routing on FIG. 67 and FIG. 66increase sensitivity to knots by concentrating a substantial length ofthe antenna in a relatively small area. FIG. 68 reduces the sensitivityto knots by avoiding meander routing, although that does force largerpads at the ends of the antenna plus folded routing to achieve somelevel of compaction.

The above three tags will be compared in the upcoming sections to thefollowing moisture tags described in earlier sections of thisdisclosure.

FIG. 69 shows a moisture tag which is a conventional compact dipole withadded interdigitated capacitor. This tag is shows very high sensitivityto moisture content and to wood imperfections.

The moisture tag of FIG. 59 which is a general-purpose dipole formounting on arbitrary surfaces and exhibits low sensitivity to moisture.The moisture tag of FIG. 61 is a modification of the moisture tag ofFIG. 59 with an added interdigitated capacitor. Similar to the MoistureTag, this tag shows high sensitivity to moisture content and to woodimperfections.

The RFID industry has settled on a particular material set andconstruction as most suitable for low-cost high-volume production. Thismaterial set consists of approximately 9 um thick aluminum on a 50-250um (2-5 mils) PET substrate with a thinner PET layer adhesively adheredcovering the aluminum and die attached to the aluminum. All threedesigns can utilize this material set and construction to achieve thebenefits of very low cost and high volume manufacturing.

FIG. 67 and FIG. 66 are shown to perform best for the pallet applicationwhile Moisture Tag and Moisture tag of FIG. 61 are overly sensitive tomoisture content. FIG. 68 is about ⅔ as sensitive to moisture content asFIG. 67 and FIG. 66 but might provide a better solution for someapplications.

A typical dielectric constant for wood at 2% moisture content is about2.5 while for 10% moisture content the dielectric constant is about 51.Simulations of FIG. 67 and FIG. 66 indicate code movement over thedielectric constant range 2-10% moisture content of about 16 code ticks(16 binary count).

The disclosure includes moisture RFID sensors that are optimized forsensing moisture on wood by providing detectable code movement in theSelf tuning engine in response to different moisture levels. Simulatedresults show that at least FIG. 67 and FIG. 66 show about 16 code tickmovement over 2-10% moisture content range and about a 5 code tickmovement over the 10%+/−2% range. The read range for the sensors isgreater than 1 m and closer to 2 m.

Embodiments further disclose the concept of extending or displacing theenvironmental disturbance closer to the tag. This enables the placementof the RFID chip at a distance from the source of the environmentalconditions to be detected and/or measured and then moving or extendingany disturbance, via for example, a tail, closer to the sensing chip forthe sensing to take place. The limitation on the distance that a tailcan detect the disturbance is a function of the environmental variablethat is to be sensed and its properties, in particular its ability to betransferred without disappearing or being diminished, as well as theproperties of the tail and its material and construction. Theseproperties, as well as the placement of the tag and tail (e.g. to takeinto effect gravity effects) also determine the time allowed for thecomplete transfer of the disturbance to the tag.

In an embodiment, in order to detect moisture at a remote location, awicking material, referred to as a tail, is used to draw the moisture(e.g. water) to the tag where it can be detected by the IC 2000. Oneadvantage of using such a wick is that the moisture can contact the tailat any or all points and that the tail can be any reasonable length andthus expands the sensing coverage area of the tag. In the currentmoisture embodiment where the moisture is water, the length of the tailis limited by the evaporation rate from the tail. The wick isessentially guaranteed to work as long as the pool of water is notexhausted and given sufficient time to travel up the tail to the tag.

Another advantage of the invention is the ability to place the tagitself in a more protected or convenient location.

The application, including placement, determines the time allowed forcomplete wicking of the water up to the tag. For example, placing thetag at a slightly lower vertical position from the end of the tail wouldutilize gravity to expedite the transfer of the moisture. Additionally,making sure the tail is monotonically descending further helps with sucha transfer or displacement.

Additionally, the invention provides for an adaptable application ofRFID tags, since, for example, the tail can be any reasonable length andconstructed from a wide variety of environmental variabledisplacing/transferring materials (e.g. water-absorbing materials for amoisture tag).

In yet another aspect of this invention, the total code movement can becontrolled by the degree of overlap between the tail and the sensingportion of the tag. For example, in an embodiment, a tag with a moisturesensing area 2 cm long could be fully covered by a tail so that when thetail has wicked moisture across its full length, the 2 cm sensingsection is fully covered producing a total code movement of N ticks fromdry to wet. Then in a second application, suppose the required codemovement is N/2, then the tail could be positioned so that it coversonly 1 cm of the sensing section.

In yet another aspect of this invention, the RFID tag and the tail couldbe affixed to each other from the factory and shipped and installed as asingle unit. In another embodiment, an installation for such an RFID tagwith a tail is done separately. This allows for a single tag that can beaffixed with various types of tails (size, length, materials). Theinstallation can be done by providing the tag and the tail with adhesivebacking and then peeling the adhesive backing from the tag andinstalling on a surface, then peeling adhesive backing from the tail andinstalling over the tag and installation surface where the sensing is tobe initiated.

In yet another embodiment, more than one type of tail can be affixed tothe same RFID tag. This would allow the same tag to sense multipleenvironmental variables.

In yet another embodiment, the more than one type of tail can be affixedto more than one Self tuning engine located on the same integratedcircuit. For example, in an embodiment one type of tail that is designedto sense one type of environmental condition can be connected to antennaport #1 (the two connections associated with Antenna Port 2502A of FIG.25A and FIG. 25B) and a different type of environmental sensingtail/antenna can be connected to antenna port #2 (the two connectionsassociated with Antenna Port 2502AN of FIG. 25A and FIG. 25B). In yetanother embodiment, a third type of tail/antenna can be connected to athird Self tuning engine on the die (e.g. Antenna Port 2502B, not shown)or a tail as to sensing ports associated for example with the ReferenceBlock 2520A of FIG. 25B where External element may comprise one or twoports. Additionally, the invention allows for the same type oftail/antenna to be connected to multiple ports in order to provideredundancy and/or additional sensing area coverage. Any combination ofthe above is part of the current invention.

In yet another embodiment, several of the same type of tail can beaffixed to the same tag and extend in several directions to increase thearea and direction of the sensing.

In an embodiment where water is to be sensed, the moisture RFID tag usesa tail that is a sheet of wicking material that is 0.1 mm thick and isthe width of the tag. The tail completely covers the tag and extendswell past the radiating edge.

Simulations as the water approaches the tag and then slowly wicks acrossthe tag highlight very progressive change in sensor code as the waterwicks along the length of the tag with negligible effect on tagsensitivity and strong total code movement of approximately 20 ticks.The simulations reflect ideal behavior of the tag. As described in thenext section, the actual tags show code sensitivity reduction for sensorcodes that are below approximately 10.

In an embodiment, the numerical experiment simulated in the previoussection is duplicated in using a saturated paper towel that is the widthof the tag and placed with the leading edge a variable distance from theradiating edge. So, the paper towel is used as the wick and it isapproximately 0.25 mm thick when saturated with water.

Measured results closely track the simulation results. That is, codemovement is progressive with approximately 20 ticks total movement withno impact on tag sensitivity for sensor codes above 10. A tagsensitivity drop is observed for sensor codes below approximately 10.Sensitivity loss for codes below approximately 10 are a known behaviorof the model of RFID integrated circuit used for this experiment, andthe behavior is independent of antenna design.

Therefore, embodiments enable a single tag to fulfill many roles such asasset tag, moisture tag and extended moisture tag (displaced/transferreddisturbance). As a product, this allows for a single SKU product withmultiple applications. Additionally, the invention provides for anadaptable application, since, for example, the tail can be anyreasonable length and constructed from a wide variety of water-absorbingmaterials. In yet another embodiment, the total code movement can belimited by not fully covering the tag with the tail. In this embodiment,the apparatus is installed by peeling an adhesive backing from the tagand installing, then peeling an adhesive backing from the tail andinstall over the tag and installation surface to the water poolinglocation.

In yet another embodiment of the wicking tails such as the onesdescribed in the previous section is explained in the current sectionand description of FIG. 70. This embodiment transmits a variable to besensed (e.g. moisture in the form of water 7002) from a remote locationto the sensing tag 7004. The basic structure as shown is paper 7006covered with tape 7008 and adhered to a surface (e.g. metal). While theadhesion can be at various points along the tail or along the entiretail, the current embodiment shows the adhesion to the surface onlyaround the boundary of the paper. The current embodiment shows thewicking of the moisture (the variable to be sensed) via capillaries(also can be referred to as arteries) through the transmitting material(in this case paper). The capillary effect is much faster at drawingwater along the tail than the water absorption in the paper. Thecapillary action is set up by placing tape 7008 over the paper 7006 andadhering the paper 7006 to the surface only around the boundary of thepaper 7006. The capillary is formed between the tape and the surface.The paper 7006 forms a spacer with thickness that varies between theedge and center, so the water can find the optimal gap for the fastestcapillary action. The water is drawn along the capillary on each edge,and then wets the paper from the sides, resulting in a U-shaped waterfront. FIG. 71 is an end view of wicking tail showing capillaryembodiment.

While the embodiments do not limit the thickness or type of material tobe used to wick the moisture to the tag, the thickness of the paper hadan effect on the flow rate of the moisture (e.g. water) up the tail. Itwas found that the thinner paper had a better flow rate than thickerpaper. Additionally, papers with ridges aligned in the direction ofwater flow had better flow rates, where the ridges provide variablespacing enabling the water to find the optimal gap for capillary action.

In another embodiment, a water (or moisture) soluble adhesive on thetape would be advantageous in some cases because it dissolves andenables the water to naturally find the optimal spacing for capillaryaction. Such a structure would be advantageous when the tags are to bedisposed of after the moisture is detected rather than being reused.Such a soluble adhesive makes the tag easier to un-install when the tagis removed. Applications include placing these tags (with the extendedtails) inside various parts of vehicle chassis to detect exposure tounwanted moisture and where these tags are optionally removed when thevehicle is being repaired due to the moisture exposure. New dry tagswith the adhesive still intact can be then installed.

The capillary tail described above is ideal for wider flat surfaces. Ifthe capillary action does not form due to non-optimal installation, thenthe water transport occurs at the absorption rate of the paper itself,and there is a tradeoff between tail length and absorption time. In yetanother embodiment, holes are periodically punched or drilled into thetape along the center axis or alternatively at other parts of the tape.This would enable the detection of moisture along the tail rather thanonly at the end. If the tape by itself is providing significantcapillary action, then it could simply wick the water to the paper tailand the operation would then proceed as described above.

Experiments were also run using four additional different embodiments.The four embodiments were the same structure as above but with:different width of paper towel for two of the embodiments, and twodifferent wicking materials from EMI Specialty Material (10 mm of 7618and 10 mm of 20535) for the other two embodiments. For theseexperiments, every sample tested took less than 90 second to exhibit 28cm of wicking.

An application of such tags described above is for the measurement ofmoisture in various parts of vehicle chassis. For example, in the trunkarea of a car and in particular the gutter areas in the trunk.

EMI Specialty Materials Paper towel Experiment 4 Experiment 7618 2053515 mm 12 mm Capillary Effect run # 10 mm 10 mm Width Width Seconds totravel 1^(st) run 89.1 46.7 37.9 37.9 Horizontal 28 cm 2^(nd) run 61.250.1 38.2 38.2

Table 1 shows results of capillary effect wicking for the fourembodiments

FIG. 72 is used to describe further experimentation on wicking materialswithin a car trunk (gutter of a vehicle body) involving an incline. Datashows the various wicking materials (three embodiments with threedifferent wicking material as shown in the Table 2) is capable ofproviding wicking for 14 cm (horizontally 11 cm+3 cm incline) in lessthan a minute illustrating sensing water in the “trunk gutter”, with thetag is placed in the gutter and the tail centered across the tag.

Smartrac Length of Experiment EMI Specialty Materials Wick absorptionrun # 7618 20535 Matl 14 cm total 1^(st) run 23 sec 40 sec 30 sec 11 cmhorizontal 2^(nd) run 50 sec 52 sec 3 cm 13 degree 3^(rd) run 40 sec 52sec 21 cm total 1^(st) run 3 min 5 sec 4 min 50 sec 2 min 50 sec 11 cmhorizontal 2^(nd) run 3 min 45 sec 3 min 52 sec 10 cm 13 degree 3^(rd)run 3 min 5 sec 3 min 28 sec 25 cm total 1^(st) run 6 min 25 sec 9 min30 sec 6 min 30 sec 11 cm horizontal 2^(nd) run 7 min 10 sec 7 min 40sec 14 cm 13 degree 3^(rd) run 7 min 15 sec 6 min 50 sec

Table 2 showing wicking experiments results with 13 degree incline

EMI Specialty Paper Models Distance Angle 7618 15026 20535   7 cm 25degrees 43 secs 45 secs 47 secs   7 cm vertical 105 sec 115 sec 105 sec  14 cm 25 degrees  5 min 15 sec  5 min 45 sec 6 min 15 sec   14 cmvertical 11 min 20 sec 11 min 10 sec 10 min 45 sec    21 cm 25 degrees 5 min 15 sec  5 min 45 sec 6 min 15 sec   21 cm vertical 43 min 30 sec35 min 20 sec 33 min 25.5 cm 25 degrees 18 min 25 sec 18 min 45 sec 18min 25.5 cm vertical >1 hr 30 mins 1 hr 13 min 1 hr 3 min

Table 3 showing wicking experiments results for vertical and 25 degreeincline

In yet other embodiments, experiments were conducted with a 25 degreeincline and a vertical incline. The results are shown in Table 3. In yetanother embodiment, another experiment was conducted with a wick stuckto a metal plate with double sided tape and the wick loose. The data wassimilar to the 25 degrees data above.

For all of the above embodiments, the RFID tag/sensor can be read inmultiple states, the calibrated neutral state (i.e. its uniqueimpedance) and the one or more states after exposure to an event. Incontrast to prior art where the RFID tag can be read in one state andthe absence of a reading is an assumption of exposure (a second state).Such prior art results in an inability to distinguish between exposureto the desired event, removal (dislodging) of tag, or tag failure. Thecurrent disclosure does not suffer from this drawback and a readingwould clearly indicate the exposure to the event and, in someembodiments, the level of exposure.

For all of the above embodiments in this section a self-tuned passiveradio frequency identification (RFID) sensor is used (see also FIGS. 3,4, 5, 6, 8, 11, 15, 20 and their associated discussions). In anotherembodiment, a conductor or transmission line couples the antenna to theprocessing module allowing the antenna to be positioned remotely oroffset from the processing module. In yet another embodiment, a sensorhaving the sensor impedance that varies with the environment may becoupled to the processing module wherein the sensor impedance may besensed via a sensor tuning module in much the same way that the antennaimpedance is sensed and since a reactive component impedance isdetermined and a value representative of the impedance is produced whichmay again be transmitted to an RFID reader for external processing.

In one embodiment, the sensor is offset from the processing module via aconductor or transmission line. In one particular embodiment, the sensoris positioned within a cavity offset from the processing module whereinthe cavity is impervious to radio frequency signals. This sensor may bean open circuited transmission line where the open circuitedtransmission line only introduces a capacitance when liquids are presentproximate to the open circuit transmission line. The capacitance changesin such an example may change with the volume of liquid proximate to theopen circuited transmission line. This is extremely useful when placingliquid or water sensors within cavities such as those contained within avehicle chassis or when the cavities are prone to fluid incursion. Thisallows the sensor to be offset from the processing module where theenvironment to be sensed is hostile to the processing module.

In another embodiment, the sensor may be an interdigitated capacitorwherein the capacitor's impedance changes in response to moisture, i.e.humidity proximate to the interdigitated capacitor. In the case of theinterdigitated capacitor, the impedance may change in response to anenvironmental dialectic constant change in the environment proximate tothe interdigitated capacitor. This may occur when different gasses orfluids proximate to the sensor involve a change in dielectric constantat the sensor as may be caused by changing gas. Thus, in one embodimentthe passive RFID sensor may be used to detect an environment toxin suchas CO, CO2, arsenic, hydrogen sulfide or other hazardous chemicals.

A change in an effective dielectric constant may involve applicationsinvolving moisture, including water vapor detection, sensing of wetmaterial stock when wetness causes product loss or deterioration,sensing of wetness in applications sensitive to mold or corrosion, anddetection of leaks in hard-to-access locations. Solid state films,having an effective dielectric constant, react to a variety of gaseswith a change in resistance or effective dielectric constant, and enablethe construction of sensor tags that respond to industrially significantgases such as CO, CO2, NOx, H2S, O2, and C12. Thin films deposited ontoan interdigitated capacitor can produce sufficient change in circuit Qto build wireless passive sensors readable through the sensor code.

In one embodiment, a non-powering event would result in a change in thecharacteristics of the antenna. An example is the fingers of the antennagetting closer to each other thus changing the impedance characteristicsof the antenna and thus the tuning frequency that a Self-tuning engineoptimizes power at. So fundamentally resulting in a code, when the RFIDtag/sensor is queried by a RFID reader or powered up by a CW signal (orpowered up in any way, e.g. via a DC or AC voltage applied to the IC)that is different than a unique calibrated code in the RFID tag that wasstored before the occurrence of the event, (e.g. at the factory, at thewarehouse, prior to including the RFID tag sensor on/in an object thatexperiences the event, when stacking object on a shelf, when object isloaded on a transporter, etc.).

Physical distortion of the antenna itself causes a change in resonantfrequency of the antenna, and the self-tuning engine can adjust a sensorcode to accommodate the change. Applications are possible for alarms,stress detection, such as for bridge integrity monitoring and inflationof flexible objects. An example of such an event is the dropping of abox that has the RFID tag/sensor affixed to or within the box.

In yet another embodiment, for sensing level of wetness, in for examplea diaper, several tags can be used to detect water level/levels ofwetness. However, using the DC ports a single tag with a long tail canbe used whose impedance will incrementally change as the level ofwetness in, for example, the diaper rises.

In another embodiment, tuning loops, antennas and/or interdigitatedcapacitors are covered with strips of adhesive material that changecolor and thus impedance with exposure to temperature, light or thelike.

Embodiments of the present disclosure allow for combining multiplesensing applications in a single die thus expanding the applicationspace of passive RFID sensors. Additional applications include altitudesensing (via pressure sensing), external accurate temperature sensing,dew point and differentials (temperature, moisture, etc.).

The passive RFID sensor may also include an RFID power harvesting moduleoperable to receive energy from the RFID reader and power the passiveRFID sensor with the received power. The processing module may determinehow much of this energy is to be consumed by the passive RFID sensor anddivert any remaining energy to a reservoir power harvesting element.Additionally, the memory module may store identification information forthe passive RFID sensor wherein the identification information may beprovided with the impedance values associated with the antenna or aseparate sensor and be provided to the RFID sensor for furtherprocessing. Additionally, a time stamp may be applied to thisinformation. This may allow the RFID reader to generate an alarm signalbased on certain measured environmental conditions.

In summary, embodiments of the present invention provide a passive RFIDmoisture tag/sensor. This moisture sensor includes one or more antennastructures having a tail. The tail is operable to transport adisturbance such as, but not limited to fluid or moisture from amonitored location wherein the antenna has an impedance and varies withproximity to the disturbance. An integrated circuit couples to theantenna structure. This IC includes a power harvesting module operableto energize the integrated circuit, an impedance-matching engine coupledto the antenna, a memory module, and a wireless communication module.The impedance-matching engine may vary a reactive component to reduce amismatch between the antenna impedance and the IC and produce animpedance value (sensor code) representative of the reactive componentimpedance. The memory module stores the impedance value (sensor code)until the wireless communication module communicates with an RFID readerand sends the impedance value/sensor code to the RFID reader. The RFIDreader may then determine an environmental condition such as thepresence of moisture or fluids at the tail of the RFID sensor. Thissensor may deploy several antenna and/or tails sensitive to uniquedisturbances. These tails may be used to monitor different locations aswell as different types of fluids. In one particular embodiment, thedisturbance is a fluid or moisture within the gutter of a vehicle body.

Thus, it is apparent that embodiments of the present disclosure haveprovided an effective and efficient method and apparatus for sensingchanges to an environment to which the RFID tag is exposed.

Those skilled in the art will recognize that modifications andvariations can be made without departing from the spirit of the presentdisclosure. Therefore, we intend that embodiments of the presentdisclosure encompass all such variations and modifications as fallwithin the scope of the appended claims. The system controllers orprocessors may comprise a microprocessor may be a single processingdevice or a plurality of processing devices. Such a processing devicemay be a microprocessor, micro-controller, digital signal processor,microcomputer, central processing unit, field programmable gate array,programmable logic device, state machine, logic circuitry, analogcircuitry, digital circuitry, and/or any device that manipulates signals(analog and/or digital) based on operational instructions. Memory maycouple to the microprocessor in the form of a single memory device or aplurality of memory devices. Such a memory device may be a read-onlymemory, random access memory, volatile memory, non-volatile memory,static memory, dynamic memory, flash memory, cache memory, and/or anydevice that stores digital information. Note that when themicroprocessor implements one or more of its functions via a statemachine, analog circuitry, digital circuitry, and/or logic circuitry,the memory storing the corresponding operational instructions may beembedded within, or external to, the circuitry comprising the statemachine, analog circuitry, digital circuitry, and/or logic circuitry.The memory stores, and the processing module executes, operationalinstructions corresponding to at least some of the steps and/orfunctions illustrated in the FIGs.

As one of average skill in the art will appreciate, the term“substantially” or “approximately”, as may be used herein, provides anindustry-accepted tolerance to its corresponding term. Such anindustry-accepted tolerance ranges from less than one percent to twentypercent and corresponds to, but is not limited to, component values,integrated circuit process variations, temperature variations, rise andfall times, and/or thermal noise. As one of average skill in the artwill further appreciate, the term “operably coupled”, as may be usedherein, includes direct coupling and indirect coupling via anothercomponent, element, circuit, or module where, for indirect coupling, theintervening component, element, circuit, or module does not modify theinformation of a signal but may adjust its current level, voltage level,and/or power level. As one of average skill in the art will alsoappreciate, inferred coupling (i.e., where one element is coupled toanother element by inference) includes direct and indirect couplingbetween two elements in the same manner as “operably coupled”. As one ofaverage skill in the art will further appreciate, the term “comparesfavorably”, as may be used herein, indicates that a comparison betweentwo or more elements, items, signals, etc., provides a desiredrelationship. For example, when the desired relationship is that signal1 has a greater magnitude than signal 2, a favorable comparison may beachieved when the magnitude of signal 1 is greater than that of signal 2or when the magnitude of signal 2 is less than that of signal 1.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the disclosure.As used herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof. The corresponding structures,materials, acts, and equivalents of all means or step plus functionelements in the claims below are intended to include any structure,material, or act for performing the function in combination with otherclaimed elements as specifically claimed. The description of the presentdisclosure has been presented for purposes of illustration anddescription, but is not intended to be exhaustive or limited to theinvention in the form disclosed. Many modifications and variations willbe apparent to those of ordinary skill in the art without departing fromthe scope and spirit of the invention. The embodiment was chosen anddescribed in order to best explain the principles of the invention andthe practical application, and to enable others of ordinary skill in theart to understand the invention for various embodiments with variousmodifications as are suited to the particular use contemplated.

What is claimed is:
 1. A radio frequency identification (RFID) tagcomprises: a power harvesting circuit operable to generate power for theRFID tag from a continuous wave of a radio frequency (RF) signal; atuning circuit that is tuned based on a capacitance setting, wherein thecapacitance setting is indicative of a power level of the power; and aprocessing module operably coupled to the tuning circuit, wherein theprocessing module is operable to generate the capacitance setting toobtain a desired power level for the power.
 2. The RFID tag of claim 1,wherein the tuning circuit comprises: an inductance associated with anantenna; and a variable capacitor operably coupled with the inductanceto form a tank circuit, wherein capacitance of the variable capacitor isset based on the capacitance setting to change a resonant frequency ofthe tank circuit.
 3. The RFID tag of claim 2, wherein the inductanceassociated with the antenna comprises one or more of: an inductor; andthe antenna.
 4. The RFID tag of claim 1 further comprises: anenvironmental sensor proximally positioned to the tuning circuit,wherein, when the environmental sensor is exposed to an environmentalcondition, a characteristic of the tuning circuit is effected, and wherethe processing module generates the capacitance setting to substantialmitigate an effect on the characteristic due to the environmentalcondition.
 5. The RFID tag of claim 4, wherein the environmental sensorcomprises: a collecting section; and a transporting section, wherein,when the RFID tag is positioned proximal to an object, at least aportion of the collecting section is within a remote area of the object.6. The RFID tag of claim 1 further comprises: a radio frequency (RF)front end, wherein the RF front end comprises: an antenna; a receiversection coupled to the antenna; and a transmitter section coupled to theantenna.
 7. The RFID tag of claim 6, wherein the processing module isfurther operable to: generate a value representative of the capacitancesetting; and transmit, via the antenna, the value within an outbound RFsignal.
 8. The RFID tag of claim 1 further comprises: a memory, whereinthe processing module is operable to generate a digital valuerepresentative of one or more of the power and the capacitance setting,and wherein the processing module stores the digital value in thememory.